close closes a file descriptor, so that it no longer refers to any file and may be reused. Any locks held on the file it was associated with, and owned by the process, are removed (regardless of the file descriptor that was used to obtain the lock).
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. SVr4 documents an additional ENOLINK error condition.
Not checking the return value of close is a common but nevertheless serious programming error. File system implementations which use techniques as ``write-behind'' to increase performance may lead to write(2) succeeding, although the data has not been written yet. The error status may be reported at a later write operation, but it is guaranteed to be reported on closing the file. Not checking the return value when closing the file may lead to silent loss of data. This can especially be observed with NFS and disk quotas.
A successful close does not guarantee that the data has been successfully saved to disk, as the kernel defers writes. It is not common for a filesystem to flush the buffers when the stream is closed. If you need to be sure that the data is physically stored use fsync(2) or sync(2), they will get you closer to that goal (it will depend on the disk hardware at this point).