A Denial-of-Service attack (also DoS attack) is an attack on a computer system or network that causes a loss of service to users. Usually it is realized through consuming all of the bandwidth available to the victim network or by overloading the computational resources of the victim system. It can be prevented by using Service Request Queue technique - limiting the number of concurrent requests one application can get while queuing all excess requests.
A Man-in-the-Middle (MitM) attack is a technique where an attack intercepts another user's session, inspects its contents and tries to modify its data or otherwise use it for malicious purposes. Measures to prevent these attachs are to use encryption of sensitive data and prevent the data being read. Some examples are using SSL, avoiding Frames/IFrames, avoid URL rewriting (SessionId is exposed).
Cross Site Scripting (XSS) is a type of computer security exploit where information from one context, where it is not trusted, can be inserted into another context, where it actually is trusted. From the trusted context, attacks can be launched.
Cross site scripting (also known as XSS) occurs when a web application gathers malicious data from a user. The data is usually gathered in the form of a hyperlink which contains malicious content within it. The user will most likely click on this link from another website, instant message, or simply just reading a web board or email message.
Usually the attacker will encode the malicious portion of the link to the site in HEX (or other encoding methods) so the request is less suspicious looking to the user when clicked on. After the data is collected by the web application, it creates an output page for the user containing the malicious data that was originally sent to it, but in a manner to make it appear as valid content from the website.
Phishing is an attempt to criminally and fraudulently acquire sensitive information, such as usernames, passwords and credit card details, by masquerading as a trustworthy entity in an electronic communication. Phishing is a social engineering technique to fool users.