Geocaching Swiss Army Knife, commonly refered to as GSAK is a GeoCaching waypoint management tool. It is a "middle man" program that allows you to manipulate the data you receive from the geocaching.com site into forms usable in the field. It will pull in your pocket queries and let you filter through them and get the data you need for a caching trip and convert it into the right form for your gps receiver and printouts, pdas, whatever.
You will need a paying (premium) membership to Geocaching.com to get the most benefit from GSAK. The membership will allow you to run pocket queries and download the results to you computer for import into GSAK. I also strongly suggest that you pay for GSAK so that Clyde (the author) can continue development, plus it will get rid of the nag screen which gets longer as time goes on.
GSAK comes with an install program that works quite well. After download, double click the GSAK######.exe file to start it. It will ask you where to install. The default subdirectory for install is \Program Files\gsak\ , but it will install elsewhere if you wish, such as \gsak\. On first startup it will want to associate file extensions .loc and .gpx with GSAK. If GSAK is the only program that you have that works with these files, then it is probably OK, but if there are other programs that you use to view or edit then you might want click cancel.