The computer's internal bus is known as the local bus, or processor bus. It provides a parallel data transfer path between the CPU and main memory and to the peripheral buses. Peripheral Buses are expansion boards (cards, adapters, etc.)that plug into the computer's bus. Signals and data pass between the peripheral device and memory. A 16-bit bus transfers two bytes at a time over 16 wires; a 32-bit bus uses 32 wires, etc. The bus is comprised of two parts: the address bus and the data bus. Addresses are sent over the address bus to signal a memory location, and the data is transferred over the data bus to that location. Various buses have been used in the PC, including the ISA, EISA, Micro Channel, VL-bus and PCI bus. Less common peripheral buses are NuBus, TURBOchannel, VMEbus, MULTIBUS and STD bus.

Popular Peripheral Buses:

PCI (Peripheral Component Interconnect)
A peripheral bus commonly used in PC. It was designed primarily by Intel and first appeared on PCs in late 1993. PCI provides a high-speed data path between the CPU and peripheral devices (video, disk, network, etc.). There are typically three or four PCI slots on the motherboard. In a Pentium PC, there is generally a mix of PCI and ISA slots or PCI and EISA slots. Early on, the PCI bus was known as a "local bus."
PCI runs at 33MHz, supports 32- and 64-bit data paths and bus mastering. The number of peripheral devices the bus can handle is based on loads, which have to do with inductance, capacitance and other electrical characteristics. Normally there are 10 loads per bus. The PCI chipset uses three, leaving seven for peripherals. Controllers built onto the motherboard use one load, whereas controllers that plug into an expansion slot use 1.5 loads.
PCI provides "plug and play" capability, automatically configuring the PCI cards at startup. When PCI is used with the ISA bus, the only thing that is generally required is to indicate in the CMOS memory which IRQs are already in use by ISA cards. PCI takes care of the rest. PCI allows IRQs to be shared, which helps to solve the problem of limited IRQs available on a PC. For example, if there were only one IRQ left over after ISA devices were given their required IRQs, all PCI devices could share it. In a PCI-only machine, there cannot be insufficient IRQs, as all can be shared.

PCMCIA or PC CARD PCMCIA stands for Personal Computer Memory Card International Association An international standards body and trade association that was founded in 1989 to establish a standard for connecting peripherals to portable computers or LAPTOPS. The PC Card is a credit-card sized, removable module for portable computers standardized by PCMCIA. PC Cards are also known as "PCMCIA cards." PC Cards are 16-bit devices that are used to attach modems, network adapters, sound cards, radio transceivers, solid state disks and hard disks to a portable computer. The PC Card is a "plug and play" device, which is configured automatically by the Card Services software.

Card and Socket Services
In order to use a PC Card slot in the computer, Card and Socket services must be loaded, typically at system startup. Card and Socket Services software is generally included with laptops that have PC Card slots. It also comes packaged with PC Cards. Card Services manage system resources required by the PC Card, and, on PCs, determines which IRQs and memory and I/O addresses are assigned. They also manage hot swapping and pass changes in events to higher-level drivers written for specific cards.

USB
(Universal Serial Bus) A hardware interface for low-speed peripherals such as the keyboard, mouse, joystick, scanner, printer and telephony devices. It also supports MPEG-1 and MPEG-2 digital video. USB has a maximum bandwidth of 1.5 Mbytes/sec, and up to 127 devices can be attached. Peripherals can be plugged in and unplugged without turning the system off. USB ports began to appear on PCs in 1997, and Windows 98 fully supports it. The devices are plugged directly into a four-pin USB socket on the PC or into a multi-port hub that plugs into the PC or into a device that also functions as a hub for other devices. USB ports on the PC and hubs use a rectangular Type A socket. All cables that are permanently attached to the device have a Type A plug.Devices that use a separate cable have a square Type B socket, and the cable that connects them has both a Type A and a Type B plug.