Decriminalisation usually means that some criminal penalties are removed, but an activity is still not "legal". Fines may still apply, and decriminalisation is usually partial (i.e. possession of drugs may be decriminalised, though you can still be fined, but dealing may still be fully criminalised).

Legalisation means going the whole hog - removing all fines and criminal penalties. This usually also means bringing an activity in the mainstream by, for example, taxing it like any other activity, or making it subject to workplace safety regulation. This is what New Zealand did with prostitution.

Wikipedia explains this reasonably well:

https://en.wikipedia.org/wiki/Decriminalization

https://en.wikipedia.org/wiki/Legalization