By rates do you mean minimum years of promotion (rate of promotion)? Or pay rates?
I'll give you a broader look at the system, if you'd like.
Officers have "ranks". Enlisted Sailors have "rates".
E-1 = Seaman Recruit
E-2 = Seaman Apprentice
E-3 = Seaman
E-4 = Petty Officer Third Class
E-5 = Petty Officer Second Class
E-6 = Petty Officer First Class
E-7 = Chief Petty Officer
E-8 = Senior Chief Petty Officer
E-9 = Master Chief Petty Officer
Additionally, specific jobs on the enlisted side are referred to as "ratings". They usually have a two or three letter designator and are denoted in the actual rate.
Here are some specific rating examples:
BM = Boatswain's Mate
PN = Personnelman
NC = Navy Counselor
MA = Master-at-Arms
CTI = Cryptologic Technician Interpretive
Let's take the "Yeoman" rating for a complete look through the rates:
YNSR = Yeoman Seaman Recruit
YNSA = Yeoman Seaman Apprentice
YNSN = Yeoman Seaman
YN3 = Yeoman Third Class
YN2 = Yeoman Second Class
YN1 = Yeoman First Class
YNC = Chief Yeoman
YNCS = Senior Chief Yeoman
YNCM = Master Chief Yeoman
In order to get a rating, enlisted personnel must complete the required initial training for their particular skill. Most of the time, this is done by attending and graduating from an "A" School. With some rare exceptions, a Sailor cannot promote above E-3 until they have completed these requirements. That's why "Petty Officers" are considered to be the same as other services' "Non-Commissioned Officers" - they've proven themselves proficient at their trade. Once the required training is completed, Sailors get a rating badge that is worn on the sleeve of the working and dress uniforms (whites or blues).
Ultimately, Enlisted Sailors can either go by the generic short titles (ex. Petty Officer, Senior Chief), or by the longer, more official ones (ex. Master Chief Boatswain's Mate, Dental Technician Third Class).
Recap:
Officers have "ranks"; Enlisted have "rates"
Enlisted jobs are "ratings"
I know this seems a bit confusing at first.....and there are yet more details that I haven't covered here, but I promise it will all be second nature by the time the test rolls around in Newport. Hope this helped - at least a little.