Network Cards may be integrated into the motherboard, but more often than not, they are add-on cards like the one pictured above.  The above pictured network card (also called ethernet cards or NICs) is a 10/100 base-T card.  The RJ-45 jack is for Category 5 and similar cables.  The small green lights indicate activity and the speed at which the network is running.  This is a PCI card.