The types of primers will change with the result we are trying to achieve and the surface that is to be painted. 

Priming is an extremely important part of painting.  Failing to prime, although not seen immediately, will eventually allow top coat paints to chip, or in spots be absorbed into the surface giving an uneven coat.

One thing we do when priming large areas of a raw surface, primarily wood,  is to add solvents or extenders which will allow the primer to stay wet longer for more absorption.

In priming something that might receive a lot of abuse or in changing one finish to another, a primer with more resins (bonders) is in order. This will give us our first shell, allowing our paint to adhere more affectively.

For interior patch work, because it will probably be primed more than once, we will use a relatively inexpensive brand.Our goal here is not to apply an undercoat of protective film, but simply to change the density allowing us to achieve an even sheen in our finish coats. This is where we use a primer with more solvents to binders for greater absorption.

There are two main reasons to prime any surface; to seal, and to gain adhesion, and both are needed to achieve a premium finish.