Re: Whats the differance between..
To put it into simpler terms the Intel Core i5 is the "middle" processor in Intel's Core line. It's not as entry-level as the i3 but not as fast as the i7. The i5 has four cores compared to the two in the i3. This gives it a better advantage in multi-threaded applications. Many apps these days are multi-threaded, it's just good sense to get more cores if possible. If the current generation of processors is the same as the last generation (Ivy Bridge), then the Core i5 doesn't have Hyper Threading technology, whereas the i3 and i7 do. Hyper Threading gives each core of the processor two threads to work with. It doesn't double performance but it makes it quicker. For the i3, it makes up for some shortcomings. In the case of the i7, it just makes it even better than good. So, to break it down:
i3: Fast enough. two cores. Hyper Threading helps with multi-threaded apps.
i5: Fast. Four cores better than two cores with Hyper Threading.
i7: Really fast. Four cores and Hyper Threading make it the fastest of the three.
In general, the i7 is just faster, and will probably feel faster long. Right away you won't notice many differences. I have an i5 from the last gen, and it's plenty darn fast still. If budget is a concern, the i5 is not a wrong choice. If money is no object get the i7 for the future proofing.
Thought that might be all you need to know. The rest is some technical stuff computer geeks (like me) care about.