Every network adapter has two interfaces, one to the computer and one to the network. The electrical interface to the computer is to a bus, which is a standardized port for high-rate data communication. The PC industry has had a long history of buses:
Industry Standard Architecture (ISA)The ISA bus, used to add cards within the computer, dates back to the original IBM personal computer of the early 1980s. It's prone to all the failings that you might imagine from an interface designed before anyone had an inkling of what people would extend the PC to do. Shortcomings of the ISA bus are that it's too slow, it's less reliable than the PCI bus, it doesn't support automatic device configuration in its original form, it interacts horribly with PC power management, and it has an unreliable automatic configuration mechanism.
You won't find 100BASE-T network interface cards for the ISA bus because it's incapable of transferring data that fast.
Peripheral Component Interconnect (PCI)The PCI bus is the standard internal adapter slot interface in computers today. The PCI bus supports automatic detection and configuration of new hardware by the operating system, simplifying both hardware and software installations. Following the recommendation of Intel and Microsoft, many manufacturers started shipping computers in 1999 with no slots except PCI, eliminating the ISA bus entirely.
Universal Serial Bus (USB)USB is an external interface standard designed in response to the problems that people encounter trying to add devices using the printer and modem (serial) ports on computers since the original IBM PC. USB can transfer data at more than 10Mbps, supports many simultaneously connected devices, and permits devices to be plugged in or disconnected without shutting down the computer. Following a relatively slow start in the late 1990s, USB exploded in popularity in 1999 with devices including network adapters, mice, keyboards, printers, digital cameras, scanners, game controllers, speakers, microphones, telephones, modems, serial and parallel ports, external CD-ROM and DVD drives, video capture ports, uninterruptible power supplies, fingerprint recognition sensors, security ID keys, and radios.
Although a faster USB version (2.0) is coming, USB network adapters are presently limited to 10Mbps operation. They're without question the easiest adapters to install, though, making them attractive for smaller systems.
Personal Computer Memory Card International Association (PCMCIA)Laptop computers present unique problems adding new hardware because their compact packaging usually precludes using the larger industry standards for desktop computers such as ISA or PCI. USB is an option for newer laptops; prior to that, the laptop industry invented a credit cardsized hardware format called PCMCIA (since renamed to PC Card). PCMCIA card network adapters plug into small bays inside laptop computers, but they often have cables vulnerable to damage at the point that the cable connects to the PCMCIA card.