A botnet is collection of computer which is remotely controlled and is known as the bots connected to the internet. These computers are compromised by the hackers or an attacker via various ways most being used is the installation of the malware software code to make the victim a bot and hence creating the botnet. The "botmaster" or "bot herder" is the computer which controls the bots with the help of standard protocols such as HTTP, IRC etc.Systems are hacked by using the various vulnerabilities or my tricking the user to run the various Trojan horse programs sent via various programs like email. These malwares generally do not have any general rule but the main outcome would be to control it remotely and perform the illegal or hacking. It will typically install modules which allow the computer to be commanded and controlled by the botnet's owner. The software might be deleted or even remain in the system till it being required by the BotMaster. The public warez networks are used to spread various malicious software in order to expand the botnet and recruit more and more new bots. Some users think that keygens are flagged as malicious software by anti-virus programs for only the illegal aspect of the software.
There are several techniques for DDOS attacks like HTTP GET and SYN floods. One of the most notorious examples of an HTTP GET attack was from the MyDoom worm, which targeted the SCO.com website. The GET attack works as its name suggests - it sends a request for a specific page (generally the homepage) to the target server. In the case of the MyDoom worm, 64 requests were sent every second from every infected system. These HTTP Get and SYN flood, codes are written in the Bot and the bot is executed on the system to execute the DDOS attack.