“Compost” is both a noun and a verb.
As a noun, it refers to decayed organic matter, which is a conversationaly acceptable term for formerly living things (plant and/or animal) that have been broken down by the feeding of bacteria and other tiny creatures into something that looks more or less like soil. This organic matter (see soil for a discussion of organic matter) is a useful addition to soil, and compost is sometimes talked about as though it were a fertilizer. While it does contain nutrients plants need, compost is really more of a soil amendment, whose primary benefit to the soil is an increase in organic matter content rather than a significant increase in the levels of particular nutrients.
The higher organic matter yields a number of benefits – higher water retention of the soil, improved retention and availability to plants of any fertilizer that you do apply, increased numbers of
soil-dwelling organisms, etc., etc.
As a verb, “to compost” or “composting” refers to the process used to make compost.
In general,this process involves mixing together a variety of food wastes, yard wastes, and/or other compounds in proportions that are favorable for the growth and reproduction of bacteria.
Within the compost “pile” or “heap” made of these materials, bacteria begin to feed and multiply.
These bacteria occur naturally on the surfaces of many living things and do not need to be added to the pile to make composting happen.
The bacteria eat and eat and reproduce and reproduce until most of the readily available nutrients are used up.
This process, which can take as little as a few weeks or as long as months (depending on how much or little you manage the pile) usually results in a substantial decrease in volume of the ingredients used to make the initial compost pile. Loss of 40-60% of the volume
of your initial pile is not uncommon.
If you manage the pile at all well, the material you end up with should look (as mentioned above) and smell more or less like soil.
Do not confuse the terms ‘compost’ and ‘mulch’.
Mulch may be composted to become useful to the garden but added to a garden with expectations of increased fertility would be wrong.