abort terminates an observation started by run and similar commands. The observation is not saved to disk. If abort is given when there is no observation then it will `succeed' but will have no effect.
If the observation is a multiple-exposure sequence (multrun etc.), abort aborts the current run and cancels the rest of the sequence. (This behaviour applies to the WFC system but not to the IDS system.)
abort
CCD readouts cannot be aborted (due to a fundemental hardware-design fault in the CCD controllers). If abort is issued during a readout, both the run command and the abort command will wait for the readout to complete, but the data will then be discarded.
The statement that `the data are not saved to disk after abort' needs qualification. Data are written to disk at various points during an observation, so some saving will usually have happened. In general, the glance file will be partially written but may not be a valid FITS-file. The glance file will not have been renamed to an archive or scratch file if abort was run before the end of CCD readout.