I'm in need of new struts, and see that there are many suspension kits available, ranging from around US$100-$200 to close to $2000 for performance/sport suspension packages. What are the advantages of more expensive suspension systems, and what features should I be aware of, and what factors should I consider when choosing what to buy?
If you don't even know what handling means or what different suspension setups do and you know for a fact that you need new shocks/struts, then just get direct replacements. The only advice I'd give is stay away from no-brands (especially if you go to places like Monroe Muffler Brake and ask for shocks) and go for something more well known but reasonably priced. KYB comes to mind. If your car happens to be Japanese or European, you may even want to consider direct OEM replacement (I might be biased here, but lately I have not been very happy with quality of parts US manufacturers put into their cars)
If you really want to know what factors play a role in suspension of a car, I suggest you pick up a book since real answer is probably going to be a lot longer than one or two paragraphs people would write here. A while ago, I read this book and thought it was great for a beginner. Not too much math, everything is in plain english but still plenty of real content that you can take away from that book.