can someone tell me if the SiS 900 Fast ethernet Adapter is a NIC?
Sure sounds like one, especially if it's got "Fast Ethernet Adapter" in the name.
and also what is the difference between a switch and a router?
A Switch will intelligently route packets between computers on a local area network.
A Router is a device that can automatically assign IPs to computers on the local area network via DHCP, share an internet connection between them using NAT, and keep pests out of the network.
and what is the name of the cable i need to connect 2 computers together with a router or switch
An Ethernet Crossover Cable