Pretty much all ATX motherboards will feature an integrated network controller, usually referred to as on-board LAN.  The chip shown here is a Vitesse VSC8201.  As is with most if not all chips, you can see the model number printed in white on the protective top surface of the chip.  This model number can be very useful when trying to find out more about a chip or when hunting for software drivers.

A Vitesse giga-bit LAN chip

The VSC8201 supports Single Port 10/100/1000BASE-T PHY and is connected to the ethernet port at the rear of the motherboard.  It can be disabled, if needed, in the BIOS set-up.  On-board LAN can be supplemented with further LAN ports simply by adding a suitable PCI network card.  Some motherboards may feature more than one on-board LAN, giving the option of different types and configurations.