Kitbashing is a specific type of conversion or modification to a model. While conversions can cover simple adjustments, trimming or changing components in some way, kitbashing is the process of combining parts from several different model kits to create something entirely new. It can be as simple as taking one head from one kit and placing it onto the body of another kit. Alternatively, parts can also be used in completely new ways such as taking the legs from a spider kit and using them to create mutated humans. 

Generally speaking, all models can be kitbashed in some way although plastic kits tend to be the easiest to work with, as are more mechanical looking models. Organic components often have very specific sizes and shapes to allow them to join with other parts from their own kit which makes mixing and matching them difficult. Things like power armour, vehicles or robots are generally much easier because ensuring the components match up exactly is not quite as important.