That sounds like such a simple question, but it seems humus is a very complicated thing if you get into the science of it. But just to keep it on a simple everyday gardeners need to know level, I think a usable definition of humus would be the very final stage of finished compost (organic matter of any sort really). Now, that also is a little more complicated than it sounds...just when is compost finished?
For me my compost is finished when it has broken down to the point where I can use it just like seed starting mix, to the point where I don't even need to screen it, it looks and feels soft and almost like soil. For me, finished compost can be stored in large containers and does not break down any further, or at least not noticeably. I have seen the definition of humus given as being the decomposed organic matter in soil, the other part of soil of course being the non-organic or mineral part.
I have included some pics of some of my current compost piles, they won't get used until later in the year, probably Oct.
This article explains it pretty well. The article is pretty long, the first part is about humus, but the entire article is about composting, I thought it was worth a read anyhow. http://www.soilandhealth.org/03sov/0302hsted/030202/03020201.html
Thank you Seedfork. That explains well. The link you gave appears very informative. I read the first few paras and came to know I was not one among a few, but among the majority, who had asked that seemingly simple question.
I compost organic kitchen waste, some dry leaves and some wood ash in small pits. Two pits, just one cubic foot. Here in this picture, the black thing is pond silt I got when I removed an old water lily plant that had overgrown. I have to remove more such from the bottom - 22 inch deep pond. When the pit is full, I change the contents to the other one and that means rotation. By that time, the other pit which I empty would be done. I am storing them in plastic bags and keeping it in a barrel with a couple of holes [for pipes]. I am not using the barrel for water now. Is this okay? I collect dry leaves and put them in onion bags hung or kept away from the ground [to prevent termite attack]. When the leaves are crispy, it has to be crushed in the bag and the small bits would pass through the holes in the bag. We can add this material wherever we want.