A Retina display has many more pixels per inch. That means that the display is sharper and can show more detail. Specifically, Retina displays are designed so that the human eye can't discern individual pixels at standard viewing distances.
That isn't the only difference between a macbook and a macbook pro, though. I like the 13" size, as kind of the sweet spot between usability and weight, and it's where there's the most variety in models, so that's what I'll cover. In 13" you have the macbook air, the macbook pro and the macbook pro with retina display.
The macbook air weighs just under 3lb. The macbook pro weighs 4.5lb, and the macbook pro with retina display weighs 3.5 lb. Go to an apple store and lift them all. Look at the sizes too - a macbook air is notably smaller than a macbook pro - not so much smaller than the pro with retina display.
The macbook pro has the most powerful processor. It will be fastest for anything that's processor-bound. It's only slightly faster than the macbook pro with retina display, but notably faster than the macbook air. If she's going to mostly be writing papers and browsing the internet, it probably doesn't matter. If she's a science, math or computer science major (compiling, modeling, statistics, etc), it's likely to make a difference. Video editing, photoshop, etc will also take a lot of processing power.
The macbook pro with retina display can take more RAM. RAM also influences speed - especially if you're doing many things at once. If she'll be working with large files (graphics editing, analyzing data sets, etc), having more RAM is a benefit. As I recall, you can't upgrade the RAM in a retina mac book pro, so unless you were planning to buy the one with 16GB of RAM - it's a moot point. The RAM in the regular macbook pro is upgradeable, so she could start with 4GB and upgrade to 8GB.
The regular macbook pro can still use a spinning disk. The air and retina can only use SSDs. An SSD is faster, and a little more reliable due to a lack of moving parts. Spinning disks are pretty reliable these days, so I'd think more about the speed than the reliability. Spinning disks can also make a little bit of noise.
Battery life - the Air gets you 12 hours of battery life. Macbook pro has 7. Retina macbook pro has 9.
I noticed some other minor differences when I was in the apple store. Things like - the keyboards are differently responsive, and have slightly different key spacing. They're subtle differences and a matter of personal preference. Some of the differences I didn't care about, and some were considerations. I count the superdrive in "minor differences" because I haven't ever needed the external superdrive we have - you might consider it a more significant factor.
I have a 13" Retina macbook pro. My second choice would have been a 13" macbook air (and I almost bought one the week before the retina macbooks came out). Battery life, size and weight mean a lot when you're carrying your laptop to class, or even when you're carrying it around the house. For most purposes, they can all be configured to be sufficiently powerful. The screens are all pretty nice. I went to the apple store 2-3 times and played around with them before I made my choice.
If you're looking for a discount, student discounts are your best bet. If your state offers a tax-free day for back to school, that's a good time to buy. I'd buy a refurbished macbook, but I know not everyone would. They get the standard 1 year warranty, and you can buy applecare on them.