Level Adjustment is a games mechanic native to Dungeons & Dragons 3rd edition, which has yet to resurface.
In essence, because different possible PC races have different levels of strength, Level Adjustment was brought in as an attempt to provide some form of balance. It started out as a system called Level Equivalent, where, during character-creation, a character of a particularly powerful race had to start a certain number of levels below everyone else in the party, and... well, that was it. This ended up being an absolutely horrible system because the characters from more powerful races would rapidly catch up in level to their party members, at which point they'd be at a long-term advantage. Level Adjustment fixed this by making characters level up at a rate that was appropriate to their effective character level; under this system, a character with a +9 level adjustment required as much XP to get from their first class level to their second as a character with a +0 LA required to get from their 10th to their 11th. This had the opposite problem of level adjustments being too steep a tax, because as the game progressed, class levels became waaay more important than racial traits.
The "Monster Class" system introduced in Savage Species doesn't replace level adjustment, nor was it intended to, contrary to the claims of certain dumbfucks who like to edit wikis without knowing what the fuck they're talking about. It did, however, allow races with level adjustments to be played from level 1, by starting out as a severely nerfed version of that race (similar to the "Lesser Drow", "Lesser Planetouched" etc. from the Player's Guide to Faerun) and then taking "levels" in their monster class instead of a normal class as they leveled up.
Getting Around Level AdjustmentEdit
Unearthed Arcana included an optional rule for buying off level adjustment. Once you were a certain level, you could spend XP to remove level adjustment forever. This was very poorly explained and involved a lot of math that most people can't be bothered to remember, and the benefits were far from obvious. However, it sort of worked out: a character with a level adjustment of +1, upon earning 6,000 experience points, would hit ECL 4 and gain a third class level. When that happens, he or she would have the option of paying 3,000 experience points to reduce their LA to +0. They'd then be an ordinary level 3 character with 3,000 XP left, hanging out with a bunch of level 4 characters with 6,000 XP. However, they'd get back up to level 4 when everyone else is still 1,000 XP away from hitting level 5, and as time went on, they'd spend less and less time a level behind everyone else, and more time at the same level as everyone else. An aasmar or tiefling who wanted to pick up Outsider Wings, and who didn't take Monk or Favored Soul as their first class level, would be able to get the wings after earning only 18,000 XP (3,000 for the buyoff and 15,000 to keep to hit level 6) instead of the 21,000 XP needed to hit ECL 7. By level 20, the character with the bought-off LA will be spending around 85% of their time at the same level as their teammates, and only 15% of their time lagging a level behind.
Pathfinder, which was based on 3rd edition, just said no to level adjustments. While the system did include powerful races, they were measurably comparable to each other using the "Race Building/RP" system, so it fell to the GM to either disallow overly powerful races to maintain group balance, but if necessary he had the tools to actually change the mechanics of a particular race.
It also featured superior mechanics for playing a monster, by essentially putting a "floor" under it equal to the party's level (so a player wanting to be a troll would have to join a fifth-level party and only gain class levels from there starting at level 1), then dopplering out as the game went one (giving the troll a free level every three-or-so levels as his monster powers become less and less relevant, until he's caught up with the rest of the party).