Low Orbit Ion Cannon (LOIC) is an open source network stress testing and denial-of-service attack application, written in C#. LOIC was initially developed by Praetox Technologies, but was later released into the public domain, and now is hosted on several open source platforms.
LOIC basically turns your computer's network connection into a firehose of garbage requests, directed towards a target web server. On its own, one computer rarely generates enough TCP, UDP, or HTTP requests at once to overwhelm a web server—garbage requests can easily ignored while legit requests for web pages are responded to as normal.
And for your clarification you will need at least 100 computers to actually do ANYTHING and its illegal.