See:
https://en.wikipedia.org/wiki/Prostitution_in_New_Zealand
http://www.business.govt.nz/worksafe/information-guidance/al...
From 5 minutes of Googling, it seems this might be a regional difference. AU/NZ references I found seemed to be different than the typical US usage, which follows what GP said.
Massachusetts recently decriminalized THC.
Possession for Personal Use
Possession of 1 ounce or less of marijuana is decriminalized and is punishable as a civil offense. If the offender is over the age of 18 they must pay a fine of $100.
Source: http://norml.org/laws/item/massachusetts-penalties-2
The AU/NZ usage seems to use "decriminalized" to mean "removing all laws against", where the US tends to use "makes it into a civil violation". We (in the US) use legalization to mean "fully legal (and possibly regulated/licensed and taxed)"
Decriminalization seems to be about making as few laws as possible concerning the matter. Legalization involves writing a bunch of laws about what you can and cannot do.
Based on the opinions of sex worker and political activist Dolores French, I have long been for decriminalization. If a legal adult wishes to take money for sexual services, I don't see why anyone should care.
I don't see any relevance to sex work though.
https://www.collegium.ethz.ch/fileadmin/autoren/pdf_papers/1... http://www.prostitutionresearch.com/FarleyVAW.pdf
Some clarification: under legalisation, sex work is controlled by the government and is legal only under certain state-specified conditions. Decriminalisation involves the removal of all prostitution-specific laws, although sex workers and sex work businesses must still operate within the laws of the land, as must any businesses.
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: