Female header is like the duct tape of electronics. Its great for connecting things together, soldering to perf-boards, sockets for wires or break-away header, etc. Adafruit go through these guys real fast, and thought that given how handy they are, they'd offer them in a pack of five!
Each pack contains five 36-pin long headers, 0.1" pitch. They're even gold plated so they wont oxidize! You can cut these down to different sizes fairly easily. First, count how many positions you want as the final piece, then pull out the next pin using a pair of pliers. Finally, cut through the empty spot with diagonal cutters. You can clean off the half-spot using a file or with the diagonal cutters.
These are 'standard' female 0.1" headers such as used on dev boards like the Arduino. If you have something with 0.1" square-pin male header on it its almost certainly going to match with these (the only exception is 'round' swiss-pin headers, but those are not as common).