Stue,
Aircraft registered in different countries have registrations that begin with a particular letter or series of letters.
As you know, all aircraft registered in the UK have registrations beginning with G.
The N refers to aircraft registered in the United States of America.
What comes after the country part seems to differ from country to country. For example in the UK 4 letters follow after the G.
In the USA, letter and numbers can follow the N (I'm not sure how many!).
Aircraft registered in Ireland have registrations beginning with EI, followed by three letters (no numbers).
I hope that helps!
dp