A botnet is a network of zombie computers controlled by a single entity. The term is a blend of the phrase "Robot Network".  Sometimes are known as a collection of software robots, or bots, that run autonomously and automatically.

Usually the zombies in use of a botnet are compromised computers running on the Microsoft operating system and sometimes Linux though it’s for it’s rare to get infected with some sort of malware.

Read More These computers communicate with other botnet machines via the Internet. Most botnets are distributed-design systems, with the botnet operator giving instructions to only a small number of machines. These machines then propagate the instructions to other compromised machines, usually via IRC. The distributed design prevents the discovery of the controlling computers. The anonymity that a botnet affords often helps the user avoid detection and possible prosecution.
Botnets are effective in performing tasks that would be impossible given only a single computer, single IP address, or a single Internet connection.
Originally, botnets are used for performing distributed denial of service attacks. designed to interfere with access to a Web site or Internet service. A common method of attack involves flooding a target server with so many communications requests that legitimate traffic can not get through However, some modern web servers have developed strategies to combat DDoS attacks, making this use of a botnet abit ineffective.

Additionally, many counter-DDoS strategies blacklist the IP addresses of attacking computers, thus exposing the botnet's machines.
The first big DDoS attack, in February 2000 took down some of the Web's most popular sites for hours, including Yahoo, CNN, eBay, Amazon.com, Buy.com, and E-Trade. Currently facebook, twitter and Google are trying to fend off a DDos attack.
As the spam market has become profitable, and, botnets are found to be an effective resource for sending spam. At any onetime ten thousand infected pc can send a simple email Furthermore, many compromised computers contain address books of email addresses which can be incorporated into the list of addresses to send spam to. Zombies that are not actively sending spam at any point in time can be configured to scrape the web looking for new email addresses to spam, adding further value to the botnet.
A secondary objective of the botnet is to find and compromise additional computers. While this is not considered a primary objective in and of itself, the expansion of the botnet via assimilation of new computers helps it perform the primary objectives more efficiently. Thus, this secondary objective is often the bulk of a botnet's tasks. Many computer networks, especially those using Microsoft Windows computers running the default settings, inherently trust other computers on the same network. Thus, a single compromised machine on such a network constitutes an attack vector against other machines on the network. Other secondary botnet objectives include website advertisement clicking, web browser toolbar installations, keylogging, and social bookmarking poll manipulation.

There is no surefire way to prevent a DDoS attack. However, a company can reduce its risk by buying plenty of servers and bandwidth, and hosting content on backup servers. Companies can also limit the number of connections that the Web server allows at any one time and set the firewall to block certain types of data that are used in DDoS attacks, said intrusecurity Uganda' David. .

In addition, companies can ask the ISP to impose bandwidth limits or block the IP addresses serving up the attack. Some companies offer DoS detection software, and website developers can configure their Web server to monitor traffic patterns and automatically ban IP addresses that could be associated with an attack.  Better yet  once an attack has been launched, a company can try to redirect the attack traffic to a null IP address, or a block it. Now the question comes back to you "Is your pc part of the Botnet?"