Hi Joey, yes there are important differences. What you get depends on your needs for performance, reliability and cost. If you specify, we could probably give you a better opinion.

Enterprise: High performance, 24/7 operation, error recovery features, will cost you your first born
NAS: Average performance, 24/7 operation, error recovery features, good price
Green & equivalents: Low performance (poor seeking, idling makes for long spin-up), power-saving, great price.
Performance: High performance, no error recovery, a little pricey

To make a long story short:

1) Do not trust your data to any brand. If it's important, you should be backing it up or have a redundant disk setup. If you don't you'll get bitten -- just a matter of when.

2) That said, once your data is safe, you have some flexibility. I have to respectfully disagree with Vertigo and recommend against buying Samsungs, Fujitsus or "Hitachi Deathstars," as they are known in many circles. Buy Japanese cars. Never buy a Japanese hard disk. WD's and Seagate's manufacturing quality have fluctuated over the years, but I've netted fewer WD deaths than Seagates over the years. I'd say Seagates generally have an average failure rate, while WDs have low failure rates. That said, check the reviews on each series. Certain models from each can be very prone to problems

3) If you choose not to take my advice regarding #1 and like to live dangerously, I'd recommend something from the WD Caviar Blue Series (balance btw price & performance). I'd recommend something from the Red series, to perhaps get a little more reliability.

4) If you have a RAID setup or backup your data regularly, then throw a Seagate Barracuda's in that puppy. I've noticed Seagates have historically excellent IO performance versus their WD equivalent. I have two and RAID 1 and am extremely happy with them. I would normally buy WD, but they've gone crazy with their prices on performance drives the last time I looked.

5) Contrary to what many tech articles say, don't worry about the lack of error recovery (WD = TLER, Seagate = ERC...same difference) on consumer drives unless you're running a complicated raid setup using a hardware RAID controller and your computer is serving a webpage or something else that needs to be running all of the time. Those features will basically let your drive limp on for a little while longer when it starts erroring out, which is symptomatic of an impending failure anyway. For a regular desktop setup, you don't need these features.