Stock photography is basically a speculation shoot, a self-assignment by a photographer who is hoping to make the images he/she captures available for sale. The opposite of "stock" would be an "assignment shoot" where (as an example) a client has hired a photographer to shoot a specific layout for a particular company where the resulting images have been shot for a pre-determined purpose and use.

As stock photography relates to Stock Models and images with people in it, in most cases if there are people in the images who can be seen clearly enough to be recognizable, a release from those people will be necessary in order for the photographer to legally lease or sell those images. There are exceptions to this, particularly in the areas of editorial and news photography. Almost always, however, clients looking for existing photographs or "stock" will want to be sure any image they lease is fully & properly released.

Example: Lets say a photographer thinks a photograph of a man and a woman walking on the beach hand in hand at sunset may sell to a travel agency for a brochure. The photographer probably wouldn't just go to the beach looking for two toned (or not) bodies & hope for them to be groomed, styled and happy to sign a full release. That photographer would be better off finding talent through Stock Models, reducing the element of chance and eliminating the possibility of wasted time and effort.  The photographer hires the talent, they all go off to the beach for the shoot, the talent gets paid, they sign a release, and everyone goes home. The photographer then edits the film and makes the best of the images available for lease.

There are countless scenarios like the one outlined above where photographers are looking for talent who will sign a release.  The photographer takes a risk in hiring the talent because some stock images never sell.  However, from the simple beach scenario  to scenes where ten or more people people are waving hands and shouting in a board room, stock photography can be a potentially lucrative way for photographers to generate revenue.  And in all cases where the photographs contain people, the photographer will require talent who will sign a full release