OK so that's a rather broad question, but I'll try to answer it concisely with the basics.
With Pro Tools HD, a PCI or PCIe card is required within the host machine. This is called the Pro Tools
HD Core Card. This card carries out the bulk of the processing on the audio, including plug-ins. It is possible to add extra cards. These are called HD Accel cards (formerly HD Process cards) and supplement the core card. Therefore, more cards = more processing power. Typically systems are expanded to include up to 3 cards (this is called HD3) but somtimes higher numbers of cards are used, usually via an expansion chassis.
Each card supports a maximum I/O of 32 channels (across two interfaces). So you can see that with an HD3 system, you could have a maximum I/O count of 96. Each card also has 9 DSP chips and can support sample rates of up to 192KHz.
In addition to the card, an interface is required. There are 4 interfaces available from Digidesign: the 96 i/o, the 96i i/o, the 192 digital i/o and the 192 i/o. Each one of these interfaces supports up to a maximum of 16 simultaneous i/o, and provides different types of connection as well as sample rates. They connect to the Core/Accel cards using the Digilink connection - a type of serial data transmission. Where two interfaces are used with one card, the first interface connects directly to the card, and the second is connected to the other interface, essentially daisy chaining them.
The only other thing needed to fire the system up is the iLok. This is a USB dongle which would contain the license to run Pro Tools HD as well as some additionaly plug-ins.
So there you have it, that's the basic jist of Pro Tools HD.
Just as a reference to how powerful it is, one card can run these plugins simultaneously:
42 x d3 compressor, 43 x URS
Neve 1081 EQ, 27 x Rennaisance EQ 6 Band, 12 x Sony Oxford Compressor/Limiter, 24 x Sony Oxford 5 Band EQ, 5 x
Line 6 Amp Farm.
The only other thing to say, in direct response to your post, is that you can use any outboard/mixer/equipment if you can connect it to one of the interfaces via analoge connection, AES/EBU, ADAT, TDIF or SPDIF.
It is also worth noting that Apogee have reverse engineered some of Digidesign's interfaces so that they can provide an interface for Pro Tools HD with their converters.
