The i7 line is essentially the best you can get (from intel). All are quad cores and those in the 900 series have hyper threading, which means there are technically 8 cores (4 logical cores, plus 4 virtual cores). Most i5's are quad cores, but do not have hyper threading, while some are even dual cores.
If you are looking at a particular computer, google the processor with it and you should see on the first line a link to intel's summary of the chip. That should help you out the most.