When you see the  datasheet of switch, if you see that it has IEEE 802.3af Standards, it means that this switch supports POE that can supply power for small device like AP,telephone with RJ45 port.

For example,this switch has the POE function:

http://www.3anetwork.com/cisco-ws-c3750x-24t-s-price_p102.html

Full form of PoE is Power over Ethernet.
The ratification of the IEEE802.3at standard this month means that compliant (PoE) devices can now transmit up to 25W over Ethernet cabling to network appliances or equipment, which is a substantial increase from the 15.4W specified under the earlier IEEE 802.3af PoE standard.
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