stock or not

I dipped a toe into stock today (results as yet un-known) and got a little confused. What makes a stock image and what doesn’t? Having a large catalogue of as yet unused images and fighting usually for arts sake I wasn’t sure if anything was stock or not. Was preparing to be branded with hot irons on my hide a good idea or should I look at this as last resort? Can I judge my photos well enough to know what will work as stock and am I happy submitting certain things to be buried under a sea of smiling happy people?