A denial of service (DoS) attack is when a criminal tries to prevent you from accessing Web information or services. An attack could target your computer and its network connection, or it could target the computers and/or networks of the sites you are attempting to visit, so that can cannot reach Web sites, your email, your banking site, etc. DoS attacks don't only apply to web services, but concern all Internet services like mail, chat, VoIP, etc.
How does a DoS attack work?
Often, an attacker will "flood" a network with information. When you type a URL for a particular Web site into your browser, you are sending a request to that site's computer server to view the page. Sometimes you may notice that the page will load slowly if there is a lot of traffic. This is because a Web site can only handle a certain number of requests at one time. An attacker will overload the Web site with requests, so you cannot visit that site. Attackers also stage DoS attacks through email. Most email accounts, especially those through a free service like Gmail, only allow you a certain amount of data in your account at any given time. So, an attacker will send large or numerous emails to an account to consume your data limit. This prevents you from receiving your actual emails.
To prevent DoS attacks, be sure to have up to date antivirus protection. Installing a firewall will also be helpful for blocking unwanted visitors or information. Visit this website for a graphical explanation of how a DoS attack works. Indeed, DoS attacks concern mainly servers offering services, but DoS attacks are frequently launched by infected machines part of a botnet.