Expansion slots are specific slots on a PC motherboard

Motherboard

In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...

 that facilitate the placement of expansion card

Expansion card

The expansion card in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard or backplane to add functionality to a computer system via the expansion bus.One edge of the expansion card holds the contacts that fit exactly into the slot...

s.

All such expansion slots are colour coded, and are built in such a way that only a specific type of card or device can interface with/ be inserted into the device.

Types

Common types of expansion slots are PCI, PCI-X

PCI-X

PCI-X, short for PCI-eXtended, is a computer bus and expansion card standard that enhances the 32-bit PCI Local Bus for higher bandwidth demanded by servers. It is a double-wide version of PCI, running at up to four times the clock speed, but is otherwise similar in electrical implementation and...

, PCI Express

PCI Express

PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...

 and AGP

Accelerated Graphics Port

The Accelerated Graphics Port is a high-speed point-to-point channel for attaching a video card to a computer's motherboard, primarily to assist in the acceleration of 3D computer graphics. Since 2004 AGP has been progressively phased out in favor of PCI Express...

.

PCI

Peripheral Controller Interface these operate at either 33 or 66 MHz and 64 bits. This utilises parallel technology.

PCI-X

This is the latest version of PCI technology operating at 66 MHZ and 64 bits. This also utilises parallel technology.

PCI Express

This had less channels for data but it was much faster than AGP technology. It was cheaper to design and build. This type of slot was much more upgradeable/expandable. It was also serial technology.

AGP

This was a designated slot reserved for graphics cards. It was utilised in a motherboard that would be used in a PC that required high-end graphics processing abilities.

AGP Pro

Used for PCs that require high re-draw capabilities. A good example would be a PC running AutoCAD. Some slots required up to 110W of dedicated power, this would be delivered using a power supply with its own MOLEX.

PCI slots

Can supply a maximum of 5V. It is important to note that a 5V card would not fit into a 3.3V slots. There are cards that can operate on two voltages they automatically default to the lower voltage.