Check out their web site. Look at IDEA as an alternate tool. These tools are used to assess properties of data sets. The list of properties to assess is as endless as your creativity. Maybe someone else has the patience to list their uses, but you should go to a seminar to really get a feel for the product.
Do the right things for the right reasons.
I just began using ACL. It's pretty useful as there are limitations on Microsoft Access and Microsoft Excel. There has always been row limitations of 65,000 but I think that has been eliminated with Microsoft Office 2007. Don't quote me though... I have not verified it.
I suggest taking the beginners foundations course if you do decide to use ACL. I spent a few days getting used to the functionalities in ACL and then went to the class. The main plus about using ACL is that if the data is not formatted correctly, ACL has the capability to reformat all of the data in the correct format before you begin your analysis.
ACL also only reads the data, so that you know you are never making changes to the original data itself.