Can anyone tell me the difference between windowservice and API.
That's a very odd question which leads me to suspect its not actually the one you want to ask...
API is Application Progammer Interface or thereabouts - as generically as possible, its a means for a developer to interface with something by means of software (the something could be another application e.g. Word, or a Web Service or the operating system or a bit of hardware).
A Windows Service is an application that runs in a particular manner on the Windows OS - very roughly - an application without an explicit UI running "in the background" launched when the OS launches (usually, but not necessarily) and independent of logged on users.
So you're comparing a particular type of application with a means of communicating between/with applications?