It's a security-based encryption key challenge, to test the security of 72-bit RC5 keys, to see if it can be cracked. 64-bit RC5 encryption was cracked successfully in 2002. It took them about 5 years to crack that code. They are less than 1% done cracking the 72-bit key, and they have been at it since the RC5-64 challenge ended in 2002 (8 years). About 0.9% through testing all possible keys in the keyspace to see if they match.
It's a distributed computing effort, and you run a client program on your machine, which fetches keys to test, then the cruncher tests each block of keys, and sends the data back to the server. Your stats are logged daily on a stats server, and it's a competitve contest to see who can crack the most keys. Faster machines will crack better, and the more computers you have running the client, the better.
RSA securities (the company that did security for Netscape) was originally hosting the contest with a $10,000 prize to the winner. The prize was to be split between distributed.net, the winner, the winner's team, and a voted charity organization. The winner got $1,000 and his team got $1,000 to split between themselves.
So, in sum, RC5-72 is a key-cracking challenge to test the strength and security of 72-bit key encryption. We all use 128-bit keys now for internet security, which are practically as safe as we can get. At the rate they are cracking RC5-72, it could take as much as 25 more years to finally crack it and find the matching key!!
