An applet is a program which is executed within another application. When embedded into an HTML-document, the applet is executed by the user's browser after it has been downloaded. Multiple applets can be embedded into a single document. Applets are commonly written in Java or Javascript.

Applets differ from full-fledged Java applications in that they are not allowed to access certain resources on the local computer, such as files and serial devices (modems, printers, etc.), and are prohibited from communicating with most other computers across a network. The current rule is that an applet can only make an Internet connection to the computer from which the applet was sent.

Web browsers, which are often equipped with Java virtual machines, can interpret applets from Web servers. Because applets are small in files size, cross-platform compatible, and highly secure (can't be used to access users' hard drives), they are ideal for small Internet applications accessible from a browser.

Web browsers can also be extended with plug-ins though these differ from applets in that they usually require manual installation and are platform-specific.

Despite Java's aim to be a "write once, run anywhere" language, the difficulty of accomodating the variety of browsers in use on the Internet has led many to abandon client-side processing in favour of server-side Java programs for which the term servlet was coined.