Full duplex

Full duplex means 2 channels where each channel goes in the opposite direction of the other. What this means is that at 100 Mbps full duplex you can send 100 Mbps of data and receive 100 Mbps of data for an aggregate of 200 Mbps. Collisions in full duplex cannot happen. Ethernet switches are full-duplex most of the time.

Forcing an interface into full duplex mode
In BSD you can see the media modes of an interface with the ifconfig command:

$ ifconfig -m $xl0 xl0: flags=8843 mtu 1500 lladdr 00:60:08:5a:86:82 media: Ethernet 100baseTX full-duplex status: no carrier supported media: media none media 10baseT media 10baseT mediaopt full-duplex media 100baseTX media 100baseTX mediaopt full-duplex media autoselect inet 172.16.2.2 netmask 0xfffffe00 broadcast 172.16.3.255 inet6 fe80::260:8ff:fe5a:8682%xl0 prefixlen 64 scopeid 0x2

To set it to full duplex you would type:

$ ifconfig $xl0 media 100baseTX mediaopt full-duplex

In Linux you'd use mii-tool to see the duplex setting of an interface.