Here are some examples of real world decision tables (slightly edited to protect the innocent).
In the above examples, the technical aspects of the decision table have been collapsed away (standard spreadsheet feature).
The rules start from row 17 (each row results in a rule). The conditions are in column C, D, E etc.. (off screen are the actions). The value in the cells are quite simple, and have meaning when looking at the headers in Row 16. Column B is just a description. It is conventional to use color to make it obvious what the different areas of the table mean.
Note that although the decision tables look like they process top down, this is not necessarily the case. Idealy, if the rules are able to be authored in such a way as order does not matter (simply as it makes maintenance easier, as rows will not need to be shifted around all the time).
As each row is a rule, the same principles apply. As the rule engine processes the facts, any rules that match may fire (some people are confused by this. It is possible to clear the agenda when a rule fires and simulate a very simple decision table where the first match exists). Also note that you can have multiple tables on the one spreadsheet (so rules can be grouped where they share common templates, yet at the end of the day they are all combined into a one rule package). Decision tables are essentially a tool to generate DRL rules automatically.