It's not just a semantic difference. Usually, decriminalization means that the behavior remains an offense, but law enforcement agencies are required to refrain from enforcing it. Legalization means that it ceases being an offense.

For example, in CA, marijuana possession of less than 1 ounce has been decriminalized. It is still an offense, but the police won't stop you. Possessions of small amounts have been defined as infractions, so you might get something akin to a traffic ticket. A recent proposition to legalize pot completely within the state failed at the polls, mainly because it was badly structured and because there was lack of clarity about the Federal response (not because of lack of support for legalization itself).