Ships often have a prefix attached to their names to indicate which type of vessel they are, or their national affiliation. This isn't enforced by any law or governing body - it's tradition, as are so many other maritime customs. Thus, there aren't any strict guidelines that define what class a vessel falls into (with some obvious exceptions, "USS" being a prime example.) Here are the most commonly-seen acronyms, some of which are no longer in broad use:

DS
Diesel Ship
FV
Fishing Vessel
HMAS
Her Majesty's Australian Ship (used exclusively by the Australian Navy)
HMCS
Her Majesty's Canadian Ship (used exclusively by the Canadian Navy)
HMS
Her Majesty's Ship (used exclusively by British Navy vessels)
MS
Motor Ship
MTS
Motor Turbine Ship
MV
Motor Vessel (this is commonly used in the US for ferries, cargo vessels, and cruise ships)
NS
Nuclear Ship
RMS
Royal Mail Ship
RV
Research Vessel (typically oceanographic science ships)
SS
Steam Ship
SSC
Semi-Submersible Craft
STR
Steamer
STV
Sail Training Vessel
TS
Training Ship
TSS
Turbine Steam Ship
USCGC
U.S. Coast Guard Cutter
USNS
United States Naval Ship (used for vessels in US Navy service but manned by civilian crew)
USS
United States Ship (used exclusively for commissioned US Navy vessels)

This naming scheme is an imprecise science, and information on it is somewhat diffuse. If you have a common acronym that you think should be added, or some additional information on one already here, let me know and I'll add it post haste.
Thanks to apatrix for HMCS and HMAS, and to frater_219 for RV! Thanks also to StrawberryFrog and gn0sis for pointing out that HMS and variants change meaning - even on existing ships - when Britain has a female monarch.

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