Files ending in AAC contain audio encoded with Advanced Audio Coding compression.
This proprietary and lossy process has been developed as the successor to the popular MP3 format. It offers higher quality, more possibilities and has been standardized as part of the MPEG II and IV standards.
There are various profiles that are intended for specific applications.
The most common is LC (Low Complexity), which is used eg on Apple iTunes and RealNetwork.
It supports sample rates from 8 to 96kHz, a wide range of bit rates (depending on the profile) and up to 48 channels.
The contents of the file may also be protected by DRM (Digital Rights Management). Then you need a corresponding, possibly paid license to view the content.
Audio data encoded with AAC can also be found in other container formats such as mp4, m4a, m4b or even 3gp. This format is also used in video containers such as mp4 and mkv.
Most audio players can play this format, but they do not always recognize the extension. Even streamed content like Internet radios often use AAC.