You want a list of your network interfaces.
To list all interfaces, whether up or down, whose drivers are loaded:
$ ifconfig -a
To list all interfaces that are up:
To list a single interface, commonly eth0:
$ ifconfig eth0
If you are not root, ifconfig might not be in your path: try /sbin/ifconfig.
When invoked with the -a option, ifconfig lists all network interfaces that are up or down, but it will miss physical interfaces whose drivers are not loaded. For example, suppose you have a box with two Ethernet cards installed (eth0 and eth1) from different manufacturers, with different drivers, but only one (eth0) is configured in Linux (i.e., there is an /etc/sysconfig/network-scripts/ifcfg-* file for it). The other interface you don't normally use. ifconfig -a will not show the second interface until you run ifconfig eth1 to load the driver.