When I try to search for games like Rock band or Sims,there are actually only 3-5 full retail games of the two franchises but vgchartz shows me 20 searche results which include DLC's and expansions,etc.

This is very annoying.There should be an option to view everything or just the full retail games or the expansion.This is just my opinion but will greatly improve site experience.

What exactly is the difference between a full retail game, and an expansion? On PCs there is a clear definition. Consoles don't really have them though.

the main point is that you shouldn't have to waist your time to saerch a game out of 20 results when you can get it right away

Then you should be more specific with your searches. There is no way to know what game you are looking for otherwise. Without a clear definition of what is a stand-alone game versus what is an expansion then the whole exercise is rather pointless.

i went to search 'The Sims' for the first Sims games and instead of just three games The Sims 1,2,3 it showed me 15 more results which were expansions.

Now how more specific can i be

Search for The Sims shows the first game as the first result. Follow up with searches for The Sims 2 and The Sims 3.

Maybe an option to exclude expansions or add-ons you check just before you do your search.
But are there enough games with major expansions to justify it?