Your mind is alawys broadcasting thoughts. These thoughts, of course, travel out to influence the world around you, and eventually return to you. (If that doesn't make you careful about what you think, I don't know what will.)
A thought form occurs when there is a whole lot of intention and energy placed in the thought. Not only does it make the thought more effective when it is sent out, but the thought will actually take the shape of the desired goal, or at least what its creator imagines the goal as. These are able to be seen on the astral and mental planes. They have actual form. Hence, thought forms.
Technically, any thought you focus on hard enough can be a thought form. Certainly any spell or ritual you do creates thought forms, as does your strong emotions such as hate, love and the like.
Kind of a basic explination. Hope that helps.