What is PoE?
Power over Ethernet... the ability to power an endpoint device such as a phone, access point, or raspberry pi over the same cable that delivers its network connection. Quite a wonderful thing if you ask me. Less cords! But, there's so many types, PoE, PoE+, 802.3af, 802.3at, active, passive, etc...
PoE, Type 1, or 802.3af
Uses 2 pairs of the cable, for a max power of 15.4W, generally used for phones, wireless access points, and basic cameras without heaters or PTZ (pan-tilt-zoom) functions.
PoE+, Type 2, ot 802.3at
Uses 2 pairs of the cable for a max power of 30Watts, used for more powerful cameras, sometimes with PTZ or heaters, and more powerful wireless access points that have more radios/antennas.
PoE++, UPOE,4P PoE, Type 3, or 802.3bt
Uses 4 pairs of the cable for a max power of 60Watts, used for high powered cameras, video conferencing systems, or building management devices. Also some of the newer WiFi6 or 802.11ax access points.
Higher Power PoE, Type 4, 802.3bt
Uses 4 pairs of the cable for a max power of 100Watts. This is pretty rare so far, but can support devices like laptops or TVs even.
Active vs Passive PoE
There are 2 types of "modes" for PoE. Active PoE, and Passive PoE. Active PoE is found in most PoE compatible devices. This is where the device will negotiate with the switch or injector to say "hey I need power, send it on over". Passive PoE devices do not negotiate, and just wait for power to be sent to them. This can be quite a frustrating thing to figure out if you aren't aware. Many back haul WiFi devices like Ubiquiti, Cambium, and others only support passive PoE. You will go to plug them into a switch that's active PoE like Cisco or MikroTik and struggle to figure out why it won't power up. In these cases you need to use a passive injector (most of the time the device will come with one), or use a switch that allows you to manually send passive power to the device. I have yet to find Cisco gear that supports passive PoE. MikroTik, and Ubiquiti both do in most of their switching gear. Luckily, Ubiquiti (and likely others) make an adapter that will turn your passive devices into active! This is a lifesaver and will save you a lot of money having to buy new switches that support passive PoE. These are the instant 803.3af adapters, and come in indoor and outdoor varieties (for example, INS‑3AF‑O‑G).
A couple other notes
The more power you push the larger your cable needs to be. For PoE Type 1 CAT5e normally will get the job done. Once you get to PoE+ and above, you better be using CAT6, or even CAT6A (maybe CAT7, it's still pretty new). One reason for this is heat. The more power you push the more the cable will heat up. This can cause fires or melting of the insulation and shorted connections.
Also, PoE can only go about 100 meters, or about 300 feet max per the Ethernet standard. Some times you might find devices that can tolerate the AMP drop at slightly further distances, but as a rule of thumb, don't plan to go over 250 feet, to give yourself extra for patch cables and service loops. There are some extenders out there, but your mileage may vary with those, especially depending on the type of device you are trying to power. They generally aren't rated for gigabit speeds either.
This was really high level and there's a lot more to PoE, but I hope this helps someone. I know it took me a while to find the difference between active and passive and what that meant!