For starters, from the online documentation:

5.0 Writing to a database file

To write to a database, a process must first acquire a SHARED lock as described above (possibly rolling back incomplete changes if there is a hot journal). After a SHARED lock is obtained, a RESERVED lock must be acquired. The RESERVED lock signals that the process intends to write to the database at some point in the future. Only one process at a time can hold a RESERVED lock. But other processes can continue to read the database while the RESERVED lock is held.

If the process that wants to write is unable to obtain a RESERVED lock, it must mean that another process already has a RESERVED lock. In that case, the write attempt fails and returns SQLITE_BUSY.

and

SQLite uses POSIX advisory locks to implement locking on Unix. On Windows it uses the LockFile(), LockFileEx(), and UnlockFile() system calls. SQLite assumes that these system calls all work as advertised. If that is not the case, then database corruption can result. One should note that POSIX advisory locking is known to be buggy or even unimplemented on many NFS implementations (including recent versions of Mac OS X) and that there are reports of locking problems for network filesystems under Windows. Your best defense is to not use SQLite for files on a network filesystem.