I've been installing a lot of POE devices recently, and the different methods for providing power over Ethernet cables can be very confusing. There are a few standards in place, and then there's a method that isn't a standard, but is widely used.
802.3af or Active PoE:
This is the oldest standard for providing power over Ethernet cables. It allows a maximum of 15.4 watts of power to be transmitted, and the devices (switch and peripheral) negotiate the amount of power and the wires on which the power is transmitted. If a device says it is PoE-compliant, that compliance is usually referring to 802.3.af.
802.3at or PoE+:
The main difference between PoE and PoE+ is the amount of power that can be transmitted. There is still negotiation to determine the amount of power and what wires it's transmitted on, but PoE+ supports up to 25.5 watts of power. Often, access points with multiple radios or higher-powered antennas require more power than 802.3af can supply.
This provides power over the Ethernet lines, but it doesn't negotiate the amount of power or the wires on which the power is sent. Many devices use Passive PoE (notably, the Ubiquiti line of network hardware often uses 24v Passive PoE) to provide power to remote devices. With Passive PoE, the proprietary nature of the power specifics means that it's often wise to use only power injectors or switches specifically designed for the devices that require Passive PoE. The power is "always on", so it's possible to burn out devices if they're not prepared for electrified Ethernet wires, or if the CAT5 cabling is wired incorrectly.
Figure 1. This AP requires a Passive PoE 24v supply. It can be confusing, because even though it says it's PoE, it won't power on using a standard 802.3af switch.
The best practice for using power over Ethernet is either to use equipment that adheres to the 802.3af/at standards or to use the power injectors or switches specifically designed for the hardware. Usually, the standard-based PoE devices are more expensive, but the ability to use any brand PoE switch and device often makes the extra expense worthwhile. That said, there's nothing wrong with Passive PoE, as long as the correct power is given to the correct devices.