ATA (aka Parallel ATA or PATA)
Use it for: Internal floppy drives, CD-ROM drives, and other disks
Add more ports by: Inserting an additional PCI controller card (each ATA port can support two devices)
Still often used to connect internal floppy drives (should your PC even have one of those) and occasionally CD-ROM drives, ATA is the analog cousin of SATA. Because it is analog, it requires you to use certain cables and/or manually set jumper pins on connected devices.
Use it for: Keyboards, mice, other input devices
It's similar in performance and use to: USB 1.0
Add more ports by: Inserting an additional PCI controller card
This old, analog serial cable is still sometimes used to connect keyboards and mice. If you're troubleshooting a PC problem, connecting such input devices might be worthwhile, especially if you're having trouble interfacing with the BIOS. Otherwise, USB input devices are much more common and will likely work just as well.
Use it for: Connecting PDAs, phones, GPS devices, digital cameras, earpieces, and other wireless audio gear; simple networking between PCs and/or gadgets; linking video game controllers
If you have a choice, select it instead of: USB (if wireless connectivity matters to you)
It's similar in performance and use to: USB
Add the functionality by: Installing an internal PC card or (more often) a USB-to-Bluetooth adapter dongle
Designed as a short-range, wireless connection for PCs and gadgets, Bluetooth can reach distances of 100 meters with certain hardware, but you'll most likely use it within a single room. The 2.1 version is currently the most widely used; it's backward-compatible with older Bluetooth devices, too. Bluetooth is a versatile connection and useful in many situations, although its modest speed of 3 megabits per second means that it won't replace other wireless tech. To connect devices, you "pair" them, putting each in a discoverable mode. You might also enter a password if both devices have a keyboard. (If only one does, consult your manual or try simple combinations such as '1234' or '0000'.)