Definition of LAG
A player who is called a LAG is an aggressive loose player. In other words, a player who plays a lot of hands very aggressively. We can label a player as LAG based on the statistics collected by our tracker. For example, a shorthanded player who plays 35/25 would qualify as a LAG. The 35 is his VPIP (Voluntary Put Money Into Pot) i.e. how often he puts money into the pot of his own free will and the 25 is the percentage of preflop raises. Here we see that the player raises more often than he calls.
LAGs and TAGs, two winning styles
Not all LAG (lousse agressif) and TAG (tight agressif) players are winners, but all winning players are either LAG or TAG. This is the style you should be playing. If you are at a table full of good players, perhaps the TAG style will be more suitable (fewer hands will be played) and if the table is very passive, the LAG style may be more suitable... Note that if you are a beginner player, the TAG style will be more suitable for you since you need to be very solid to be able to profit from a LAG style.