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A Table and its contents are stored on disk as a directory with the Table name (e.g. 3C273XC1.ms) and may contain a number of files. One technical aspect of Tables that you should be aware of is that some Tables reference other Tables. We try to avoid making copies of Tables so that if two Tables contain almost the same data except for one or two columns, then a reference Table is created that points back to the original Table for shared information. This mechanism is also used for sorts and selections. If you sort or select a Table to create another Table then the new Table points back to the original Table via a column of row numbers. The only reason you need to be aware of this is to avoid deleting the original Table.
Although there can be many different types of Tables in AIPS++, representing different types of data, there are two important types of Tables which you will commonly use: MeasurementSet (MS) (which represents data from a telescope, e.g. visibility data) and Image (which represents the distributions of intensity on the sky). An Image may have two or more dimensions, e.g. position, velocity, stokes.