Ifconfig, when invoked with no arguments will display all the details of currently active interfaces.
If you give the interface name as an argument, the details of that specific interface will be displayed.
Example:
# ifconfig eth0
Display Details of All interfaces Including Disabled Interfaces
# ifconfig -a
Disable an interface
# ifconfig eth0 down
Enable an interface
# ifconfig eth0 up
Change MTU
# ifconfig eth0 mtu XX