The pack files are managed by the game loader and contain all the game assets. In the graphics pack are all the models, textures, and animations, while the sound pack contains all the game sounds. There is also experimental pack content which is meant for use while on the test server.
The game loader manages the updating and patching of the pack files and must complete this process before you can log in. Often, this is the reason why the play button is ghosted out, and clicking on the pack updates button will allow you to see your progress. As patching can involve downloading a number of MB of data, it is suggested that bringing up the game loader before you need it is a good idea. This reduces the chance of having to wait while download and patching happens, when you could otherwise be playing.
The packs are Black Box, and it is preferred that they are left alone. Any identified attempts to modify the pack counts via client modification will result in bans if found to be guilty. The game's terms and conditions are clear on this. Essentially, it counts as hacking the client, so don't do it.