An EthernetBridge is used to connect networks together. It works at the data link layer, taking all traffic from one side and transmitting it across the other, bridging the network segments.
If you are too cheap, or poor, to buy a switch and you have spare patch (crossover) cables and network cards it relatively straight forward to network three, or more, PCs together by using the middle PCs as bridges.
iface eth0 inet manual
iface eth1 inet manual