The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) on each port. Only 12.95 W is assured to be available at the powered device as some power dissipates in the cable.
The updated IEEE 802.3at-2009 PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power. The 2009 standard prohibits a powered device from using all four pairs for power.
MORE. Clipped from table on the page referenced in additional notes.
802.3af (802.3at Type 1) "PoE" 802.3at Type 2 "PoE+"
Voltage range (at PSE) 44.0–57.0 V 50.0–57.0 V
Voltage range (at PD) 37.0–57.0 V 42.5–57.0 V
If a device that requires 24V is connected to a PoE port one or both devices will probably be damaged due to over current condition set up by the difference in voltage requirements and delivery.