The following is drawn from this site, the last paragraph is the relevant bit.
Network cables, such as Cat 5e and Cat 6, comprise eight wires
arranged as four twisted pairs. In 10 and 100BASE-T Ethernet, two of
these pairs are used for sending information, and these are known as
the data pairs. The other two pairs are unused and are referred to as
the spare pairs (Gigabit Ethernet uses all four pairs).
Because electrical currents flow in a loop, two conductors are
required to deliver power over a cable. PoE treats each pair as a
single conductor, and can use either the two data pairs or the two
spare pairs to carry electrical current.
Power over Ethernet is injected onto the cable at a voltage between 44
and 57 volts DC, and typically 48 volts is used. This relatively high
voltage allows efficient power transfer along the cable, while still
being low enough to be regarded as safe.
This voltage is safe for users, but it can still damage equipment that
has not been designed to receive PoE. Therefore, before a PoE switch
or midspan (known as a PSE, for power sourcing equipment) can enable
power to a connected IP camera or other equipment (known as a PD, for
powered device), it must perform a signature detection process.
Signature detection uses a lower voltage to detect a characteristic
signature of IEEE-compatible PDs (a 25kOhm resistance). Once this
signature has been detected, the PSE knows that higher voltages can be