Hibiscus is a complex plant, a member of the Malvaceae family and has more than 200 known species.
The best known species is Hibiscus rosa-sinensis, also known as tropical or Chinese hibiscus. It originates from the Indian Ocean area in East Asia. Its original color was red, but now this genus includes many hybrids of different colors. The hibiscus flower grows darker as the plant ages. It is the most popular in subtropical and tropical areas and it is the national flower of Malaysia. In Tahiti, it is traditional for women to wear a red hibiscus flower behind their ear.
One of the most interesting characteristic of this particular species it is about its genetics; it is a polyploid. It has a difficult definition and the explanations are technical, but in simple terms, the baby-plant's flower can be different from the parent's because H. rosa-sinensis has more than two sets of chromosomes, unlike other species. This characteristic allowed crossing and re-crossing varieties, so that many new and amazing varieties have resulted.
That might also be the reason why some of the same hibiscus blooms are different when blooming outdoor from those blooming indoor. My pink hibiscus had two different blooms last winter when blooming indoor, each looking like two different 'Cajun' hybrids.
'Stolen Kiss' 'Madame Dupont'
The amazing 'Cajun' hybrid series started to win the hibiscus lovers' hearts a few years ago. There are over 80 hybrids in this series; each is more amazing than the last, not only because of their colors, but also because of their cute names. These are a few examples, but you can read and see more in this article on Cajun Hibiscus at Dupont Nursery written by Lisa Neal (aka "jusmelisa").
Cajun 'Acadian Spring' and H.schizopetalus 'Bluebird' 'Love Pat'
'Voodoo Queen' 'Mystic Pink' 'Florida sunset'
'Tsunami' 'Black Jack' 'C'est bon'
These are a few H. rosa-sinensis hybrids grown by our DG friends.
'Snow Queen' 'Heavy Metal' 'Tamibon'
'El capitolio Bloody Mary' 'Flaming wind' 'Evangeline'
'Fort Myers Yellow' 'Painted Lady' 'Painted Lady'
'Susan Schlueter' No. 1 Hibiscus 2006 'Yoda' 'Yoda'
'Magdalena's delight' 'Kona Double Pink' 'Pink blush'
Hibiscus rosa-sinensis has many medicinal uses in Chinese herbology and it is used in India for haircare purposes. They are making shampoos and extracts from the leaves and flowers for hair coloring, anti dandruff and preventing hair loss. Hibiscus tea is good for lowering blood pressure. Some of the species are used as ingredient for herbal teas. Hibiscus cannabinus known as kenaf is used in paper-making.
The flowers contain substances that can reduce fever, relieve pain, inhibit spasm activities and work well as an antioxidant. The roots are used to cure coughs. Complex extracts of hibiscus can reduce skin cancer, inhibit the Herpes virus and reduce cholesterol.
Scientists have also discovered it might have good results in increasing male and female fertility.
The most popular species in the temperate zones is Hibiscus syriacus, the hardy garden hibiscus, also known as Althea or Rose of Sharon. It is the national flower of South Korea.
In our country it is often used to create hedges. They are trimmed back during summer, twice before blooming and once more in the fall.
'Blue Satin' 'Hamabo' 'Lavender Chiffon'
'Minerva' 'Shintaeyang' 'Woodbridge'
Hibiscus moscheutos, also known as the swamp rose mallow, is a perennial, cold-hardy hibiscus species, growing very well in wetlands and along rivers. Propagation is made from seeds and cuttings.
I found one of these on the field next to our house, growing wild like a small bush. The leaves are different from other hibiscus species, looking more like maple leaves. Flowers are pink and it can be propagate by seeds or cuttings. It was used to make hardy hybrids like Hibiscus coccineus, H.laevis, H. militaris and H. palustris.
Hibiscus coccineus, also known as Swamp Hibiscus, Scarlet Rose Mallow, Texas Star, Scarlett Hibiscus, is native in the U.S. and hardy in zones 6-11.
Lately, hibiscus breeders have succeded in obtaining tropical hardy hybrids between Hibiscus rosa-sinensis, Hibiscus moscheutos, Hibiscus coccineus and Hibiscus laevis. Three of these hybrids are reported to be hardy down to 30F (-34C): 'Angelique', 'Cherub' and 'Pink Cornet'. I know I'd like one of those! They are available from local garden centers and mail order nurseries.
These are a few of the Hibiscus moscheutos hybrids.
'Lord Baltimore' 'Luna Pink Swirl' 'Luna Red'
'Lady Baltimore' 'Luna Blush' 'Fireball'
'Plum Crazy' 'Disco Belle Pink' 'Fantasia'
'Pink Clouds' 'Kopper King' 'Pink Dinner Plate'
'Red Southern Bell' 'Luna red' seed pod
Hibiscus diversifolius, also known as "swamp hibiscus," is very widespread in its native habitat of New Zealand, Australia and in swampy coastal areas in Africa. Its name indicates it has different shape of leaves on the same plant. I was amazed to find it has yellow flowers with a maroon basal spot, similar to a few other hibiscus species and some looking almost identical to it.
Hibiscus sabdariffa, the 'Jamaican cocktail' (also known as Roselle) is an annual herbaceous tropical bush. Its flowers are similar to cotton - Gossypium. It originates from India to Malaysia, but it came to US from Jamaica where it is also known as "Flor de Jamaica."
It is widely used in the Carribean for food as a vegetable and for making teas and jam of its fruits (calyces), which are similar to cranberries in taste. 
Hibiscus trionum is maybe the shortest hibiscus and it is an annual, invasive plant. It is native to southern Europe, where grows as a weed, but also as cultivated in the gardens. It grows in the U.S., too. The flower produces pods with dozens of seeds which are spread all over the garden and pop up the next summer. I have it all over in my garden and until now, I thought it was a poppy. Although the flowers are nice, I had to pull them off, so they won't take over the garden. It is also called the "flower of an hour." That challenges me to watch the flowers and see if they are really staying opened for an hour. I need to experiment more with this plant, but perhaps I can control its growth by trimming it back. The stalk appears to be very thin and tends to crawl on the ground.
Related to hibiscus, also from the Malvaceae family, are okra, cotton and hollyhock.
Okra (Abelmoschus esculentus syn. Hibiscus esculentus), has flowers almost identical to Hibiscus trionum. It is native to West Africa and best known in U.S. as an ingredient in gumbo. The fruits are used as vegetables and make very tasty meals. These veggies are also very appreciated in Europe, including in my country, Romania.
Another edible okra, Abelmoschus manihot, also called Hibiscus manihot or sunset hibiscus. Its leaves and blossoms are used for wrapping food specialties or for salads.
Abelmoschus moschatus (syn. Hibiscus abelmoschus) is a frost-hardy plant, also called the Musk okra. It is a medicinal and aromatic plant, native to India. The seeds have the heavy frangrance of musk. It is used for culinary purposes. The seeds are added to coffee, while the unripe pods, leaves and new shoots are used as vegetables. It also serves to add flavor to liquers, such as Benedictine. As a medicinal plant, it is used to relieve spasms and digestive tract, cramp, poor circulation and aching joints. It can be used as insecticide and an aphrodisiac too. 
According to some sources, Abutilon is not from the same family as Hibiscus, but others include it in the Malvaceae family. The bloom is similar to hibiscus, with the same mallow family-type of staminal column, but it is smaller and looks like a lantern, thus the name "Chinese lantern." It is native to Hawaii, but now is listed as endangered. Abutilon makes a beautiful, ornamental plant when grown indoors in pots or outdoors in mild zones. I have it in my home and it blooms almost all year long.
The Hawaiian hibiscus encompass seven species. Some of these are very rare in nature and many are listed as endangered. The mostly grown in the islands, as ornamental plants, are the H. rosa-sinensis hybrids. The state flower of Hawaii is a yellow Hibiscus brackenridgei (ma'o hau hele). 
Malvaviscus is a flowering plant related to Hibiscus, also in the Malvaceae family. It is also called turk's-cap mallow, sleeping hibiscus and marzapan. It differs from the Hibiscus genera because the fruit is divided into five parts.
Hibiscus schizopetalus, also called Japanese lantern or waltzing ladies, has hanging red flowers with feathery petals. It is native to eastern Africa in Kenya, Tanzania and Mozambique and one of the ancestors of many hibiscus hybrids.
Alcea, the hollyhock, is a perennial, tall-growing plant, with only single stalks full of blooms having the appearance of a Hibiscus moscheutos. The flower is similar to hibiscus, and comes in many colors.
Now that you have seen all these beauties, aren't you feeling great, even if you've had a bad day? Imagine having them in your home and garden, they will heal your soul with their beauty and their positive energy!
Many thanks for our DG friends who graciously allowed me to use their pictures : luvsgrtdanes, rutholive, Sharran, SingingWolf, scardiecat, marsue, 9kittymom, boojum, eyesoftexas, Early_Bloomer, Marylyn_Tx, wren, BeaHive, wareagle, eltel, kathy_ann, catherinejat, Snug_As_Bug_Rug, DebbiesDaisy.
,  Wikipedia article on Hibiscus
 Purdue University