To hide the diversity of equipment that may be used in a networking environment, TCP/IP defines an abstract interface through which the hardware is accessed. This interface offers a set of operations which is the same for all types of hardware and basically deals with sending and receiving packets.
For each periphereal device you want to use for networking, a corresponding interface has to be present in the kernel. For example, Ethernet interfaces in are called eth0 and eth1, and SLIP interfaces come as sl0, sl1, etc. These interface names are used for configuration purposes when you want to name a particular physical device to the kernel. They have no meaning beyond that.
To be useable for TCP/IP networking, an interface must be assigned an IP address which serves as its identifcation when communicating with the rest of the world. This address is different from the interface name mentioned above; if you compare an interface to door, then the address is like the name-plate pinned on it.
Of course, there are other device parameters that may be set; one of these is the maximum size of datagrams that can be processed by that particular piece of hardware, also called Maximum Transfer Unit, or MTU. Other attributes will be introduced later.