It depends on the system actually. Are you playing 3.x or 4e?

3.5: Not qualified. Let someone else answer.

4e: In 4e, an encounter of a party's level is x monsters( where x is the number of players) of n level( where n is the level of the party.

So a level four party of four PCs would have a "standard" encounter with 4 Monsters of Exp level 4. Of course, you can change this by making the encounter easier(-2 Party level) or harder(+2 Party level) or by changing standard monsters into elites(1 Elite = 2 standard) or Solos(1 Solo = 4 standard) or by adding Minions(4 Minions = 1 standard).

There are tables in the DMG that spell it out a little more clearly.