Why do I need Firewire and what is it?

This is a mirror of the original tip that was found on Apple's website.  The tip is now maintained on the mirror by macmaps.com.

The image below shows a Mac that has both types of Firewire found on Apple's computers:

Firewire ports are rectangular with angular notches on one side.  Firewire 800 ports are rectangular but narrower than modem or ethernet ports and are almost square

Where the ports labelled Firewire with no number, are 400 Mbps Firewire 400, also known as IEEE 1394a.  Firewire 800 is known as 1394b and goes up to 800 Mbps.

Some Mac models don't come with Firewire, while others do not. Be careful to read the specs of the Mac you are to buy. Firewire 800 and Firewire 400 are both easy to interface with each other using a Firewire 400 to Firewire 800 adapter.  For Macs of these two port types, they require a 6 pin and 9 pin Firewire connector respectively.

Note: starting in 2011 and with the release of Mac OS X 10.7, and new computers in that year, Thunderbolt has gradually replaced the need for Firewire, and adapting to it from Thunderbolt is also possible.

Firewire allows the following advantages which you won't find via other interfaces for the Mac, except Thunderbolt and USB-C Macs:

1. Target Disk Mode. This allows a Mac to be treated as an external hard drive to another Mac. It makes it easier to recover data from a Mac that becomes unbootable, if you don't have access to another drive, but do have access to another Mac with Firewire from a store, friend, or usergroup.  This feature was never available for USB 1 to 3, but
became available on USB-C, when it was released on the Mac in 2015 on some Mac models.   Thunderbolt ports also support it.
Two ports look like Thunderbolt on Macs, but only one is genuine, look for the thunderbolt next to the port:
The thunderbolt port has the lightning icon next to it.
USB-C looks like
A USB-C port has a symentric elongated oval.

2. Uncompressed video capture. While USB 2 camcorders exist, they all compress their video capture to go through USB 2.

3. Faster data transfer to third party peripherals and drives. In spite of USB 2's rated 480 Mbps (60 MBps), the speed of USB on most machines never exceeds 150 Mbps. Firewire in its original 1394a standard is still a full 400 Mbps.  USB 3 is faster, but so is Thunderbolt.   Thunderbolt firewire adapters

exist, and so do USB-C to thunderbolt.

4. No networking configuration is needed for sharing data between Macs using Firewire in target disc mode.

5. PowerPC Macs in Mac OS X could only boot externally off Firewire drives. This is not a limitation of Intel Macs.

6. Firewire IP allows networking over Firewire at full Firewire speeds. This technique is faster than all networking except Gigabit ethernet and fiberoptic methods available by expanding the Mac tower and server configurations. It is documented in the Mac OS X 10.4 help documents, and available
to all versions of Mac OS X.

On the other hand, USB does offer these advantages:

1. Up to 127 items can be daisy chained, instead of 63 for Firewire.

2. Many more peripherals are compatible with USB than Firewire, and are typically cheaper than the Firewire peripherals offering the same functions.

As of the writing of this tip, the following Macs do not have Firewire:

1. The October 20, 2009 MacBook.
2. The October 14, 2008 Aluminum Unibody MacBook. Note: the MacBook White
available simultaneously with the Aluminum Unibody MacBook does have Firewire.
All MacBook Pros have Firewire.
3. All MacBook Airs.
4. iMacs 350 Mhz Indigo from July 19th 2000 to February 22nd
5. iMacs pre October 5, 1999 that were not DV.
6. Powerbook G3 Bronze keyboard (a.k.a. Lombard), discontinued February 16, 2000
7. PowerMac G3 Beige. The PowerMac G3 Blue and White, and G4 PCI had Firewire, but lacked Target Disk Mode support, or Mac OS X booting support via Firewire. All AGP PowerMac G4s and newer had support for this. To differentiate the G4 models, see this article:

8. The only color iBooks that had Firewire are easily identifiable by this port configuration in this image .

Shows the side of an iBook G3 DV.   From rear to front along the side is a modem port, ethernet port, USB port, Firewire port and headphone jack

9. 2012 and later Macs.   All 2011 Macs had Firewire.   

Note the second port from the right is a Firewire port. All white iBooks have Firewire.

Older Macs do not have Firewire built-in.