This page gives a better introduction: http://dangerousprototypes.com/docs/Bus_Pirate

Basically a tool to interface with whatever IC you might have. Flexible input supporting many protocols, just hook it up to a random chip or serial debug pins on a board you are reverse-engineering and you can figure out the signal mapping and protocol decoding in software.

Piggybacking on that question, could I use this to interrogate a device over rs485 and reverse engineer the protocol it speaks? In particular, a pool heater and a variable speed pool pump

It can be used as a TTL serial adapter (ex getting a command line on common hardware/IoT devices), JTAG (debug, dump firmware), reading flash memory chips, etc

If you care for such things, you can use the Bus Pirate to see the speed of your code .. I have a macro I can wrap any function in, and by way of GPIO toggles, see on my bus pirate/logic analyzer, how many milliseconds/nanoseconds certain functions take to execute .. its also quite useful for getting a console - I have loaned my BP to a friend, who used it to reset his electric mopeds firmware ..