What Is a Denial of Service Attack?
A Denial of Service (DoS) Attack is an attack that interrupts network availability when users require its service. It is an attack that is launched against network servers. Such an attack can be intentional (a malicious act) to deliberately shut down a network and prevent users from access to the resources they need, or accidental (when the hardware or software fails). Simply said, it targets the network server (or another network segment) to make it unavailable to users.
The Virtualpune glossary states this: "Denial of service attack is an act by the criminal, who floods the bandwidth of the victim’s network or fills his e-mail box with spam mail depriving him of the services he is entitled to access." 
Types of Denial of Service Attacks (attacks that can disrupt service) include:
Transmission failures. From a malicious standpoint, such an attack can cause an overload of rejected data.
Flooding. When more data is sent by the attacker than can be handled by the network, connection flooding is the outcome.
Syn Flood. This attack will send more requests to a server than it can handle. Too many SYN requests sent by an attacker with no ACKs (acknowledgments) can fill the victim's SYN_RECV queue.
Note: Regardless of what type of attack occurs (from those listed above), they all have one thing in common: all are availability type attacks that threaten continued network service to users. They cause the server to process too many system requests and, as a result, the network is effectively blocked and is unavailable.
DoS attacks continue to be a problem today. There are, however, ways computer users can stop or reduce them.