The Filesystem API abstracts out the functionality needed for reading and writing local files to the filesystem to be done securely, on a variety of host types.
The WP_Filesystem is capable of detecting when the users for written files will not match, and switches to a method using FTP or similar instead. Depending on the available PHP libraries, WP_Filesystem supports three different methods of using FTP (via extension, sockets, or over-SSH) and will automatically choose the correct method.
Reference:
Filesystem API (Codex)
