The net80211 layer provides functionality required by wireless cards. The code is meant to be shared between FreeBSD and NetBSD, and you should try to keep NetBSD-specific bits in the source file ieee80211_netbsd.c (likewise, there's ieee80211_freebsd.c in FreeBSD).
For more information about the ieee80211 interfaces, see Chapter 9 (Kernel Internals) of the NetBSD manual pages at http://www.netbsd.org/docs/.
The responsibilities of the net80211 layer are as follows:
The ieee80211 layer positions itself logically between the device driver and the ethernet module, although for transmission it's called indirectly by the device driver instead of control passing straight through it. For input, the ieee80211 layer receives packets from the device driver, strips any information useful only to wireless devices, and in case of data payload proceeds to hand the Ethernet frame up to ether_input.