You have to choose a source for your data (Under Database > Database Expert).
Your source can be a myriad of data types but I generally use SQL DBs at my shop.
So you connect to the SQL and you can then expand the data source options to use the tables, views or stored procedures from that SQL DB. So if you have rights within your system you can create a SQL view (or sp)joining your tables via your WHERE clause. That view is now available as your source instead of the 2 tables which need to be joined via crystals less flexible means. This (or stored procs which also allow for parameters) is an EXTREMELY useful process as you can do all your data manipulation and joining on the server instead of crystal. Much faster and much more flexible in SQL.
The COMMAND option is within crystal and lets you write SQL style select and joins. After you connect to the DB (same as above) there is an 'Add Command' option which opens a window where you can write your SQL command to grab your data. I have found COmmands to be very slow although others have not, so it may have to do with my environment.
Does that help or do you need more?
Edited by DBlank - 26 Aug 2010 at 9:16am