From Mac Guides
Ethernet ports are used to connect computers to a network (such as a local network or the internet) or occasionally peripherals such as laser printers or hard disks. Commonly, a router will be used to split an internet connection between many computers and to create a LAN. Ethernet ports were standard on all Macintosh computers beginning with the PowerMac G3 (Beige), and available as third-party addons for most earlier machines.
Ethernet ports are measured in 10, 10/100, or 10/100/1000 Base-T. The number represents the theoretical speed of the connection, measured in Mbps. They have a theoretical maximum of 1.2MB/second, 12MB/second and 120MB second for 10 Base-T, 100 Base-T and 1000 Base-T respectively.
All Intel Macs to date include Gigabit (1000Mbps) ethernet. The Mac Pro (formerly the PowerMac), MacBook Pro (formerly the Powerbook) have had Gigabit Ethernet since July 2000 and October 2001 respectively. Of the consumer Macs the iMac was the first to get Gigabit ethernet with the Rev B iMac G5 released in May 2005. The Mac Mini and MacBook (formerly the iBook) gained Gigabit Ethernet upon becoming Intel Machines in February and May 2006 respectively.
Note that the Powerbook 12" controversially never had Gigabit Ethernet.