Applet
An applet is a small program that runs in the context of a larger program on a client computer. This usually refers to Java applets, which run in a browser. A similar program on a server is called a servlet.
Macromedia Flash is another widespread applet nowadays that provides motion graphics, interaction and sound.
Applets tend to have some form of user interface. This distinguishes them from a program written in a scripting programming language (such as JavaScript) that also runs in the context of a larger, client program, but which would not be considered an applet.
Applets generally have the capability of interacting with and/or influencing their host program, although they are generally not required to do so.
