I get the difference between SCSI and IDE drives. But now I’m seeing SATA drives floating around. (Not literally, but you get the idea). What is a SATA drive, and can it play well with IDE interfaces, or not? And what is EIDE, specifically?
Simply put, SATA (Serial ATA) drives use a new, high-speed interface to get data from the disk to the computer much faster than IDE. If your computer only has IDE, you’ll need an expansion card in order to use SATA. EIDE is Enhanced IDE, and is backward compatible with the older interface, unlike SATA.
SATA interface is faster, but most of the drives don’t yet have the speed to come near maxing it out, so there is not a huge performance gain.
The S in SATA is for serial. With only a few wires, you end up with a fairly thin round cable, instead of the wide flat ribbons of IDE and EIDE. Those wide flat cables were a pain to route neatly, and even the most anal installation would still obstruct airflow significantly. Thus the main benifit to SATA drives is reduced installation labor, and better cooling for the machine.
SATA interface is faster, but most of the drives don’t yet have the speed to come near maxing it out, so there is not a huge performance gain.
The S in SATA is for serial. With only a few wires, you end up with a fairly thin round cable, instead of the wide flat ribbons of IDE and EIDE. Those wide flat cables were a pain to route neatly, and even the most anal installation would still obstruct airflow significantly. Thus the main benifit to SATA drives is reduced installation labor, and better cooling for the machine.
For example, Kevbo is posting in parallel, not serial 
EIDE/IDE/ATA connectors (the terms are used mostly interchangably now, even though there are some distinctions) are about an inch an a half long, two rows of pins with one missing as a guide marker, connected to a flat cable as mentioned. Each cable supports two drives, and there are generally two connectors on the motherboard, so your “average” IDE system will support four IDE drives (generally hard drives and CD-ROMS, floppies use a different connector). Traditionally these drives use the older-style power connectors (white plastic, four large holes in a row).
SATA connectors are much smaller, a flat “blade” a little over a half inch long, with a small kink on one side, like a tall, skinny “L”. There aren’t many standards about how many are on a motherboard; my last machine had 1, the new computer has five plus a couple externals. SATA can be used as an external hookup as well, but you don’t see many of them yet (either the external drives or the external hookups). SATA has it’s own style power connector (much flatter, smaller holes, usually black plastic), but almost every SATA drive I’ve seen will take an IDE power connector as well.
If your computer/motherboard is less than a year or so old, it’s almost certainly got both connectors on the board. If it’s less than 2 years old, there’s a good chance it has at least one SATA connector on there (it will be labled SATA on the board, but they’re small connectors and can be hard to see), but it’s not certain.
As the Constitution apparently requires, SATA taps are always placed at the least convenient location on the motherboard, so if you’re looking for one, be sure to look under things like expansion cards and along the edges of the board.
I think some motherboards are better at placing SATA connectors than others. I have an Asus A8N32 Premium which has 5 internal connectors and three external (two at the back). Only two of the internal connectors are placed inconveniently, though; the other three are below and somewhat to the side of the IDE headers.
The main advantages of SATA are, as mentioned, speed, smaller cables (better air flow), and up to 128 devices can be connected, but also they can be hot-pluggable, so external SATA drives can be hooked up like external USB and FireWire devices.