Expansion cards are hardware components that can be added to a computer to enhance its capabilities. These cards fit into slots on the computer's motherboard and can provide additional features such as improved graphics, sound, or network connectivity. Common types of expansion cards include graphics cards, sound cards, and network interface cards. By installing an expansion card, users can upgrade their systems without replacing the entire computer. This modular approach allows for greater customization and performance improvements, making it easier to keep up with evolving technology and software requirements.