JavaScript is a programming language used to implement additional functions in HTML. JavaScript makes it possible to e.g. check input fields or display calendars.

There are also negative examples of JavaScript use (e.g. pop-up windows that common pop-up blockers can't block anymore). Many browsers offer the possibility to deactivate JavaScript but this also limits the user's possibilities a lot.

JavaScript was developed for fast execution and is integrated in the browsers. Crawlers usually can't interpret JavaScript code and search engines ignore it. Particularly from an SEO point of view this is an important thing to keep in mind.