They are used for storage of images or spectra. They are basically the same as keywords just associated to data files instead of the monitor. They are used for storage of heterogeneous data contrary to frames and masks which store homogeneous data. They are typically used for saving lists of e.g. stellar positions and magnitudes. See Chapter 5 for a full description of the table facilities.
Many commands output their results or take their input from tables.
See Chapter 3 for further details.