Low Orbit Ion Cannon
"LOIC" stands for "Low Orbit Ion Cannon".
The program itself doesn't control any botnets or the like (at least in its original iterations), but is intended to be used in very large groups, often organized in IRC channels. It was originally written to work only in Windows but has now been adapted to work on other platforms as well. Newer versions also include an option to connect to an IRC server and have users of a channel control the client software to further automate attacks.
It is not advisable to use this software, as it facilitates DDOS attacks, which are considered illegal by the FBI and other law enforcement agencies.