OK
Here's my pick at the subject:
As all of the others have already posted, Pro Tools is a system composed by Hardware and software, both produced by the same company, Digidesign, and engineered to work together; you can run several other softwares on Digi's hardware (in both native and TDM versions, like Logic, Digital Performer, Vegas, Nuendo, Wavelab), by using the appropriate drivers, but you cannot run the PT software on non-Digi's hardware, that's it.
Depending on the caliber of work you are in to do you can go from native Pro Tools systems (those that rely on your computer's horsepower to manage audio processing and playback/recording operations) like the Toolbox XP, 001, 002 or 002Rack or TDM/HD systems; the pros of native configurations are: less expensive than a TDM system, allow pro results if used by a smart operator, almost plug & play (with a bit of optimization of the computer); the cons are limited features (like track count and A/V sync options) compared to TDM, no way to expand I/O (you cannot add interfaces to go beyond the phisical limit of say the 18 I/Os of a 001 or 002), some plug ins (not much though) are only available in TDM version and not RTAS (Real Time Audio Suite, the plug-in format for the native platform).
TDM/HDTDM systems are modular, so you can configure one to fit your own situations, starting from just the CPU, a DSP card and one interface to full blown 128 tracks - 96 I/O.
The biggest difference (apart from cost, a HDTDM rig starts at $12.000 without computer, Hard drives and various options) is that TDM systems rely on DSP cards to process and handle audio (and MIDI) with no hit on the computer's CPU, this can be an andvantage when dealing with large sessions with lots of tracks/plug ins; the more DSP (now called process cards) you add the more power you have, but power hat ist cost (and it's not low).
My advice would be as suggested by Michael to log on the Digidesign website and check each product out to understand which one can fit you needs/budget.
I currently work in my project studio with a 001 setup and I'm able to transfer session to a nerby TDM equipped studio where I can complete the projects on their bigger rig if I need to.
I have to say that for the majority of the projects I do the 32 tracks of the Pro Tools LE 5.3.1 version I'm using are pretty much all that I need, and with the DSP of my Athlon PC I rarely run out of power for plug ins, and this brings me to the next issue:
It's now possible to say that Pro Tools can be efficiently run on a PC Windows XP machine too: I self built mine (using the invaluable help and info from many of the guys on the forum at Digi, BTW you can find me there too) and I can have the same power of a Mac G4 that would've cost me twice as much, DON'T GET ME WRONG, Macs are great, I'm not shredding, but you can definitely have an alternative on them provided that you have the patience to do some research on the right configuration (see the forum at Digi, check [
="http://duc.digidesign.com/showflat.php?Cat=&Board=UBB32&Number=360675&page=0&view=collapsed&sb=5&o=7&fpart=1"]THIS THREAD[/
] out) and are willing/able to put one together and maintain it (no rocket science, I did it so can you); you can't just get a run of the mill PC and have it working as a DAW without problems; if you don't want to do that get a Mac (an used G4 466 should be OK) which comes almost set right out of the box; either way you'll be fine ; the feature is almost equal now between Macs and PCs, and evenplug-insdeveloper are writing PC versions of theirplug-ins
I hope this helps, and excuse me if I've been a bit long but such a topic can't be explained in two words.
L.G.
