A powered hub uses the power provided by a Wall wart to power USB devices instead of the computers power. In the case of the Raspberry Pi, a powered hub is necessary to power many devices such as hard drives, CD drives, etc. because powering all these devices would leave the Raspberry Pi under powered, leading to shorter component life and even a non functioning Pi. When looking for a good powered hub you want to look for the following:
Durability: You want to look at the construction of the hub itself, if you can see imperfections such as a warped side, or chips DON'T GET IT. If it already has these imperfections and is still brand new, this shows poor quality control. Which will later lead to more problems.
Number of Ports: The number of ports does matter, depending on the number of devices you want to power. You will want to get a hub that has enough ports to support all your devices.
Power Source: Many USB hubs will have a USB cable with 2 USB A ends on it. You DO NOT want this. These kinds of powered hubs get their power from the computer, not the wall. When looking for hubs You want to make sure that the wall wart is included.
Here is a list of the five Best USB Hubs, although I recommend the first two for you application.
Here is the one I use, It is a Raspberry Pi themed hub that works great and is VERY durable.