The definitions you can find on the internet for stock photography sound like this
Used widely by creative professionals in need of ready-made images that illustrate a specific lifestyle, scene, mood, or process
or

Photographs that are bought by a company and stored in a database. These photographs are licensed to publishers, advertising agencies
Since this is the photography forum, we must have some photographers around.

How many of you started taking stock photography and what systems do you use for selling them ?