Marijuana is not chemically addictive.  It does not contain nicotine like people seem to think.
Marijuana contains a substance called THC.  When consumed, it induces a psychoactive state during which short-term effects are episodic memory loss and other minor psychological effects.
Long-term effects are less clear.  Some suggest that consistent use leads to short-term memory loss and may be more likely to develop psychosis.  Other information suggest no significant negative long-term side-effects.