Ground rent is the money that you pay as a leaseholder to the Landlord or Freeholder for basically renting the land that the leasehold property sits on. This is usually a fixed sum and is paid annually.
The service charge is payment for all the services (such as maintenance of gardens and communual areas) that you as a leaseholder will use but will not be specifically responsible for. This will often be an estimate of what the landlord will anticipate will be for the forthcoming year.
Ground rent can be collected directly by the Landlord, by the managing agent or a specialist ground rent collection company on behalf of the Landlord or Freeholder