Instrument Rating: an ICAO rating, valid anywhere in the world. Allows you to fly in cloud or poor vis, out of sight of the surface, in all types of airspace. The JAR implementation of this rating is particularly difficult to get - in particular, the ground exams will be very difficult if you are working full time. Alternatives to the JAR IR are an FAA IR (which is only valid on N-registered aircraft), or an IMC Rating (with restrictions as described below).
IMC Rating: This is a UK-only rating. It is not recognised, and therefore can't be used, anywhere in the world except the UK. It allows you to fly in cloud or poor vis, out of sight of the surface, in class D, E, F and G airspace in the UK. It allows you to fly instrument approaches, but the recommended minima for these approaches are far higher than for an Instrument Rated pilot (500' absolutely minimum for a precision approach, 600' for non-precision, but an individual approach may have higher limits).
Hope that clears up the confusion!
FFF
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