EPR = Engine Perssure Ratio. In lay mans terms, the means the difference between the pressuer of the incoming air to the pressure of the outgoing burned gasses in a jet engine, or the diffenende between the pressure of the air in the inlet section of a compressor and the compressor outlet pressure, depending on the particular application. EPR is a ratio, so naturally it has no units. EPR can directly give an accurate indication of the engines' thrust, so is used in some, but but all fan and jet engines.
As to the second part of your question, N1 relates to Low Pressure spool of a multi spool jet engine, N2 relates to the second spool(on Rolls Royce RB211 engines there are 3 spools, GE and PW tend to use just 2 spools), N3 for the third spool etc. N1 is the fan and low pressure compressor, AND the low pressure turbine, so you can deduce from this that a SPOOL consists of a compressor and a turbine section, connected by a shaft.
I hope that this is of some help, but if not, then some rotter has nicked my screen name and password, and it isn't really me posting this!
Cheers JFD