PCs have always had ports, connections for peripherals such as a printer or an external modem, that allow data to flow both ways. Early PCs had serial and parallel ports, connection types which existed before PCs did, and in the old days you connected your noisy tractor-feed dot-matrix printer to the parallel port and your 2400bps external modem to a serial port, and you were ready to dial up the BBS and annoy people. There were also game ports for joysticks and gamepads.
People also sometimes want to temporarily connect two PCs directly to each other to transfer data. Back when the only ready alternative was diskettes, there were methods for doing that using the serial and parallel ports, notably the third-party product LapLink which included software and cables, and the DOS and Windows utilities INTERLNK and Direct Cable Connect.
Conventional serial ports at 230 Kbps are still acceptable for connecting external V.90 modems for dialup service, because the data transfer rate is limited by the 56 Kbps nominal speed of the modem-to-modem connection over the phone line. We have other gadgets now that would like to go a lot faster.
There are two newer types of high-speed ports commonly seen: USB and FireWire (IEEE 1394). Both port types are used to connect fast peripherals and sometimes to temporarily connect two PCs. Early USB 1.0/1.1 ports had data rates of 1.5-12 Mbps and were already faster than serial and parallel ports. Common USB ports now support USB 2.0; USB 1.1 ports are only seen on older hardware. In 2009 I bought a cheap USB numeric keypad with hub ports, that turned out to be USB 1.1.
Apparently Apple is now producing Macs with USB ports and without FireWire ports, so FireWire may be on its way out. Peripherals such as printers, external modems, and joysticks/gamepads, that used to use the older parallel, serial, and game port types, are now all made to connect to USB ports.
eSATA (external SATA) is a modification for external connections of the SATA hard disk interface, allowing you to connect to a SATA hard disk that's outside the case, in an external enclosure. It's very fast, but pretty much only seen in the form of an aftermarket option for desktop PCs, so far.
Nominal transfer rates:
It would appear at first glance that USB 2.0 is a little faster than FireWire, but there are architecture differences to be aware of.
USB is a host-based or master/slave protocol: one end of a USB connection is always the boss. USB connections that need to behave like peer-to-peer, such as cables for directly connecting two PCs, require a little adapter circuitry. FireWire is fundamentally a peer-to-peer protocol; devices on the ends of a FireWire link can negotiate data handling. In practice FireWire400 will be faster than USB 2.0, especially for sustained throughput such as with an external hard disk.
There's also an 800-Mbps version of FireWire, referred to as FireWire800 or IEEE 1394b, and the FireWire spec provides for up to 1600 Mbps.
One advantage USB retains is that nearly every Windows PC you encounter will already have USB 2.0 ports, as shipped from the manufacturer, usually including handy front-panel ports. FireWire ports are mostly seen on digital camcorders, Apple Macs and iPods, Sony PCs, some PCs designed specifically for audio/video production, and as aftermarket add-ons to other PCs. A USB connection also always supplies power as well as data, which some FireWire connection types don't. There were a few peripherals around for a while that used both a FireWire and a USB connection, with USB only supplying power.
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