Client-side scripting generally refers to the class of computer programs on the web that are executed client-side, by the user's web browser, instead of server-side (on the web server). This type of computer programming is an important part of the Dynamic HTML (DHTML) concept, enabling web pages to be scripted; that is, to have different and changing content depending on user input, environmental conditions (such as the time of day), or other variables.

Client-side scripts have greater access to the information and functions available on the user's computer, whereas server-side scripts have greater access to the information and functions available on the server. Server-side scripts require that their language's interpreter is installed on the server, and produce the same output regardless of the client's browser, operating system, or other system details.

Client-side scripts do not require additional software on the server (making them popular with authors who lack administrative access to their servers); however, they do require that the user's web browser understands the scripting language in which they are written. It is therefore impractical for an author to write scripts in a language that is not supported by the web browsers used by a majority of his or her audience.

Client-side scripts are often embedded within an HTML document, but they may also be contained in a separate file, which is referenced by the document (or documents) that use it. Upon request, the necessary files are sent to the user's computer by the web server (or servers) on which they reside.

The user's web browser executes the script, then displays the document, including any visible output from the script. Client-side scripts may also contain instructions for the browser to follow if the user interacts with the document in a certain way, e.g., clicks a certain button. These instructions can be followed without further communication with the server, though they may require such communication.