Addon cards, also known as expansion cards, are additional hardware components that can be installed into specific slots on a computer's motherboard or other compatible devices to expand their functionality. These cards are a way to enhance the capabilities of a computer or electronic device beyond what is already integrated into its main components.

Most addon cards are designed to fit into one of the various types of expansion slots available on a computer's motherboard. The most common types of expansion slots include:

1. PCI (Peripheral Component Interconnect): PCI slots were prevalent in older computers and allowed for a wide range of expansion cards, including sound cards, network cards, and various other interfaces.

2. PCIe (Peripheral Component Interconnect Express): PCIe slots have replaced PCI in modern computers due to their significantly higher data transfer rates. PCIe allows for faster communication between the motherboard and the addon cards. Modern graphics cards, network cards, and storage controllers primarily use PCIe slots.

3. AGP (Accelerated Graphics Port): AGP was a dedicated slot for graphics cards in older systems, but it has been replaced by PCIe for modern computers.

Examples of addon cards include:

1. Graphics Cards (GPU): These addon cards are dedicated to handling graphical processing and rendering images, videos, and 3D graphics. They are essential for gaming, video editing, and other graphics-intensive tasks.

2. Sound Cards: Sound cards enhance the audio capabilities of a computer by providing improved audio input and output. They can improve sound quality for gaming, music production, and multimedia applications.

3. Network Interface Cards (NIC): NIC addon cards enable a computer to connect to a local area network (LAN) or the internet. They are useful when a motherboard's built-in network port is not sufficient or malfunctions.

4. RAID Controllers: RAID (Redundant Array of Independent Disks) controllers allow multiple hard drives to be combined into arrays to improve data redundancy, performance, or both.

5. USB Expansion Cards: These cards provide additional USB ports for connecting more peripherals and devices.

6. WiFi Cards: These addon cards add wireless networking capabilities to computers that lack built-in WiFi connectivity.

7. TV Tuner Cards: TV tuner cards allow computers to receive and display television signals, often used for watching and recording TV programs.

It's important to ensure compatibility between the addon card and the motherboard or device before making a purchase. Additionally, some newer computers may have limited expansion options, as more features are integrated directly into the motherboard or replaced by external devices like USB peripherals.