They plug into any available peripheral slot on the motherboard and contain the physical interface for a specific connector type. See the illustrations. The 32 bit card is shorter. Many computers now have NIC's built in.
Laptops sometimes use cards that look like small floppies.
No two cards will have exactly the same address, thus every computer in the world will have a unique address. In Windows 95, you can find the address of an installed card. The address is a series of hexadecimal numbers. They are not so meaningful to us but they are very important to the network. Provides the physical interface between the computer and the network transmission media.