You could have both !
NICs [network interface cards] are either an on board chip or by way of
an expansion card [PCI], both types have an identical squarish outlet on
the rear of your PC housing, this is an RJ45 intererface and is used
for connecting to ADSL modems/routers etc via a cat5 ethernet cable.
NOTE you may also have an internal modem which has a similar looking
outlet [RJ11] for your telephone cable to plug into if you have/had
dialup internet. On first glance it is easy to confuse them but the
NIC's RJ45 interface is larger, so even though the telephone line will
plug into the RJ45 socket, it would be impossible to insert the RJ45
connector into the telephone socket of an internal modem.
rgds
Roberto