What is a service?
A Service is a special type of application that is available on Windows NT/2000/XP only and runs in the "background". Services can have special privileges that regular applications do not. For instance, on a limited user account a service could access certain parts of the Windows Registry that a regular application could not access.
Most Windows Services do not have a user interface and are loaded when Windows starts.
Are services dangerous?
Just like any application, a service could be written to perform malicious tasks on your computer.