Decriminalize means that it's only a misdemeanor - meaning no jail time, even though it's still illegal. However, even in states where marijuana is decriminalized, selling it, or having large quantities is still a felony, and you can go to jail.

Legalizing means it's legal! If marijuana were legalized, it would be like alcohol - legal to buy, sell, and use, although there would be restrictions on age, smoking in public, and driving while under the influence of marijuana (again, like alcohol).