NAT (Network Address Translation): Translates between public IP address, such as the one being used by a cable modem at home, to IP addresses on the internal network. Hides internal devices; without it, internal devices would each need their own public IP address.
DHCP (Dynamic Host Configuration Protocol): Provides devices with IP addresses and TCP/IP configuration information like the address of the DNS server and the address of the router that provides a path to the internet. The router address is known as the default gateway.
LAN (Local Area Network): A group of computers in the same geographic location.
Mbps, Gbps (megabit, gigabits per second): Bits transferred per second. Wired LANs today are 100 Mbps or 1,000 Mbps (1 Gbps). Bits, lowercase b, are for network speeds. Bytes, uppercase B, are for RAM capacity (4 GB) and other measurements.
AD DS (Active Directory Domain Services): Hosts user and computer objects in one place instead of having each workstation store them as they would in a Workgroup scenario. This is done by promoting a server to a domain controller. Users then have single sign-on to any computer in the network, although access can be restricted as needed.