What is the AppData folder in Windows 8 ?

The AppData folder contains app settings, files, and data specific to the apps on your PC.

The folder is hidden by default in File Explorer, and  has three hidden sub-folders: Local, LocalLow, and Roaming.

• Roaming. This folder (%appdata%) contains data that can move with your user profile from
  PC to PC—like when you’re on a domain—because this data has the ability to sync with a server.
  For example, if you sign in to a different PC on a domain, your web browser favorites or bookmarks will be available.
• Local. This folder (%localappdata%) contains data that can’t move with your user profile.
  This data is typically specific to a PC or too large to sync with a server. For example, web browsers
  usually store their temporary files here.
• LocalLow. This folder (%appdata%/…/locallow) contains data that can’t move, but also has a
  lower level of access. For example, if you’re running a web browser in a protected or safe mode,
  the app will only be able access data from the LocalLow folder.

The apps themselves choose whether to save to the Local, LocalLow, or Roaming folders. Mostdesktop
apps use the Roaming folder by default, while most Windows Store apps use the Local folder by default.

Caution:
It is not recommended for moving, deleting, or making any changes to files or folders in the
AppData folder. Doing so could make your apps run poorly or stop working entirely.