IMO this is a good on-line tutorial about Windows services.
Extract from that tutorial:
Windows Services, formerly NT Services, were introduced as a part of the Windows NT operating system. They are not available on Windows 9x and Windows Me. You need to use one of the operating systems in the NT generation such as Windows NT, Windows 2000 Professional, or Windows 2000 Server to run a Windows Service. Examples of Windows Services are server products such as Microsoft Exchange, SQL Server, and other applications such as Windows Time that sets the computer clock.