I know more or less how the GPS system works. But I would like to understand it in more details. Exactly what data is sent from a GPS satellite to a GPS receiver?

I think there is multiple types of data, since when I start up a gps receiver, it takes some time before ut becomes useful. As what I know this is because the receiver waits on some data that aren't received very often. I guess that the GPS signal that is used for the position is sent more often, but I don't know.

Is there anyone that can explain this in more detail? Or have a useful link?