Seti at Home is a project that involves many (over 1.000.000) people all over the world that donate their machines (unused) CPU time to make calculations on radio frequencies. Those radio signals are caught from the Arecibo Radio Telescope and embrace a vast portion of sky (and still a little part of the universe) and hundreds of thousand frequencies.

When you connect to the Seti@Home server, it send you a portion of a signal and your computer starts to analize to find a possible non-natural/usual signal; it search for high power signal that would carry a message. When your computer finishes to analize its portion of data it reconnects to the server on the internet and download a new workunit and the process restarts.

Your computer is not able to recognize a true message. It does only analize the workunit to find spikes and gaussians through mathematical functions; the results are collected by the server for a further analisys (the will soon begun) that will correlate the discovered candidate signals to check if they are messages or natural radio emissions from space.

I personally take care of donating unused time (mostly over the night) of that workstations and servers under my control at the university (and at home) through a set of dedicated scripts.