The most common meaning of Cookie on the Internet refers to a piece of information sent by a Web Server to a Web Browser. The Browser software is expected to save this information and send it back to the Server whenever the browser makes additional requests from the Server.
Depending on the type of Cookie used and the Browser's settings, the Browser may accept or not accept the Cookie, and may save the Cookie for either a short time or a long time.
Cookies might contain information such as login or registration information, online "shopping cart" information, user preferences, etc.
When a Server receives a request from a Browser that includes a Cookie, the Server is able to use the information stored in the Cookie. For example, the Server might customize what is sent back to the user, or keep a log of particular user's requests and may save the Cookie for either a short time or a long time.