Both USB, and PS/2 are used for serial communication, and are very similar.. The largest difference is the PS/2 bandwidth is much slower.
Also see this section on the USB wikipedia article
Mice and keyboards are frequently
fitted with USB connectors, but
because most PC motherboards still
retain PS/2 connectors for the
keyboard and mouse as of 2007, they
are often supplied with a small
USB-to-PS/2 adaptor, allowing use with
either USB or PS/2 interface. There is
no logic inside these adaptors: they
make use of the fact that such HIDs
are equipped with controllers that are
capable of serving both the USB and
the PS/2 protocol, and automatically
detect which type of port they are
plugged into.plugged into.