Sound Cards: A sound card is an expansion card or IC for producing sound on a computer that can be heard through speakers or headphones. Although the computer does not need a sound device to function, they are included on every machine in one form or another, either in an expansion slot or built into the motherboard (onboard).
Video cards: a video card is an expansion card that connects to a computer motherboard. It is used to create a picture on a display; without a video card, you would not be able to see this page. More plainly, it is a piece of hardware inside of your computer that deals with processing images and video, along with some of the tasks that the CPU is normally responsible for. Video cards are used by gamer in place of integrated graphics due to their extra processing power and video RAM.
Network Cards: A network interface controller is a computer hardware component that connects a computer to a computer network. A computer uses a network interface card (NIC) to become part of a network. The NIC contains the electronic circuitry required to communicate using a wired connection (ex: Ethernet) or a wireless connection (ex: WiFi). A network interface card is also known as a network interface controller, network adapter, or Local Area Network (LAN) adapter. The most widely used network connection for personal computers is an Ethernet connection. Ethernet is really a standard for computer network technologies that describes both hardware and communication protocols. Since Ethernet is so widely used, most modern computers have a NIC built into the motherboard. A separate network card is not required unless some other type of network is used. A Network Interface Card is required to be present in the PC to communicate over a LAN. A switch or a Hub may be required or may not be required depending on the network configuration. For example, if you have 10Base2 (Thinnet) network, you may not need a Hub or a Switch. Router is required only when there is routing involved.