All Pro Tools systems are hardware/software combinations (except for Pro Tools Free). They won't run without the proper hardware. Pro Tools will only interface with it's own hardware I/O (except the M-Audio version, which will interface with M-Audio products). So you can't just run it through your sh!tty sound card (thank god).
Pro Tools LE can do nearly everything that HD can do, but it uses your computer's CPU instead of dedicated hardware to run plug-ins. Most modern computers are fast enough that this isn't a problem.
HD has far superior I/O hardware than LE system, though. Better convertors, better clocks, better preamps, etc. But an HD3 system can run between $13K and $20K, retail.
Some people prefer other programs, but they don't offer anything that Pro Tools doesn't, and Pro Tools is pretty much the only option for professional applications because it's what everyone uses and you need to be able to go from one system to another. With Pro Tools, you can track some stuff in a big studio, some stuff at home in your closet, some stuff on the road. You can FTP sessions between band members to work on whether they are. Good stuff.
edit: You can use Pro Tools to create and edit samples, though there are other programs dedicated to this task that might be easier (Sound Forge?). Pro Tools is primarily the world's most powerful tape machine and editing station, and it just happens to do MIDI and sample editing and stuff, too. It will also interface with Reason on the same computer, so Reason's synth sessions can play back within Pro Tools along with your real audio.
edit 2: Sound engineers love to bitch about Pro Tools because it doesn't sound like analog tape. They are correct, it doesn't. But it does sound pretty good, and it's sound is more reliable and predictable than tape. So it's sort of apples and oranges in the sound department.