THC (tetrahydrocannabinol) is the active ingredient in marijuana and is stored in fat cells; because fat cells metabolize THC more slowly than muscle cells would, THC is not fully metabolized out of your system for much longer than other drugs. On a drug screening, marijuana will show up for up to 30 days on a urine test and 4 months or more on hair analysis (depending of the length of your hair). Because more and more employers are requiring employees to submit to randomized or pre-employment testing, you have to be careful with what you're putting in your system.
Also, because marijuana is metabolized more slowly, you can experience a pot 'hangover' 2-3 days after the last time you used. These hangovers will make staying alert more difficult, reduce your coordination, and decrease your depth perception (necessary for driving and a lot of sports).