On Windows, services are processes running regardless of users logging in. They provide access and features to different programs or processes. On Mac OS X, services are menu bar and context menu items available from any application that can act on a user's selection, like text or files.
Questions with this tag should mention the operating system, as Mac OS X and Windows use the term services for very different concepts. Additionally, different services may act differently on different Windows versions. Windows services can include File Sharing, Network access, or just updaters.
For Windows 2000, XP, 2003, Vista, 2008 and 7, you can access the services that start automatically through the
SERVICES.MSC Microsoft Management Console applet. This is usually accessed by pressing Windows+R to bring up the Run dialog, then typing
services.msc and pressing Enter.
DISABLED means the service is unavailable to users and processes, and must be enabled with the
MANUAL means the service can be controlled by the application, or another process requesting it. It may not automatically start up, unless another service or program requests access to it.
AUTOMATIC and AUTOMATIC (Delayed) means the service will run automatically on system boot, and can be configured by any program with access to it.
Mac OS X
Service are scripts and programs that can process selected text, files, or URLs. They can be accessed from the Services menu item in the menu bar's application menu, or from the context menu of selected text, files, or similar items.
They are often provided by installed applications to integrate their functionality with other programs, such as Safari's Search With [Default Search Provider] (acting on text selection) or Stickies' Make New Sticky Note (also acting on text selection). Users can create custom services in Automator.