i5 vs. i7 MBP Refurb
i7 processors differ
I assume - since you are comparing the i5 to the i7 - that you are thinking of the 13'' MBP models. The 15''-17'' MBP models offer only i7 processors.
The i7 processors of the 15''17'' MBP are quad core, while both the i7 and i5 in the 13'' MBP are dual core.
2 important things to notice

Due to hyper-threading one can also talk of 2 physical cores (4 logical cores) and 4 physical cores (8 logical cores).
The clock speed you usually see is just the base frequency. The introduced Intel Turbo Boost technology allows the cores to dynamically clock at higher speeds. However, not all cores at once.

Comparison chart
An offical Intel documentation including all features of all 4 processors can be found here. The two processors offered in the 13'' MBP are on the right. Differences are highlighted yellow.