A table is a systematic arrangement of data usually in rows and columns for ready reference. Sometimes this data is words, sometimes numbers and words. A PDF table must always have header columns and sometimes will have header rows. Tables can be the most complicated of PDF tags to work with and can take a lot of time and effort. If you are lucky the tables will already be tagged as tables when you get the file, or when you add tags to the file.
One of the biggest changes from Acrobat 8 Professional to Acrobat 9 Professional is with the way they handle tables. In Acrobat 8, in order to set column and row spans you’d have to go into the cell properties. In Acrobat 9 you are able to make changes to column and row spans through the table editor (was table inspector in Acrobat 8). You can also set id tags this way for complicated tables.
I’ve broken the table discussion into two pages — one for Acrobat 8 and one for Acrobat 9. Reading both would be redundant, but might be interesting for comparison.