You are developing a web-based application, and you want to ensure that an attacker cannot exploit it in an effort to steal information from the browsers of other people visiting the same site.
When you are generating HTML that must contain external input, be sure to escape that input so that if it contains embedded HTML tags, the tags are not treated as HTML by the browser.
Cross-site scripting attacks (often called CSS, but more frequently XSS in an effort to avoid confusion with cascading style sheets) are a general class of attacks with a common root cause: insufficient input validation. The goal of many cross-site scripting attacks is to steal information (usually the contents of some specific cookie) from unsuspecting users. Other times, the goal is to get an unsuspecting user to launch an attack on himself. These attacks are especially a problem for sites that store sensitive information, such as login data or session IDs, in cookies. Cookie theft could allow an attacker to hijack a session or glean other information that is intended to be private.
Consider, for example, a web-based message board, where many different people visit the site to read the messages that other people have posted, and to post messages themselves. When someone posts a new message to the board, if the message board software does not properly validate the input, the message could contain malicious HTML that, when viewed by other people, ...