I think they test for levels of a certain type of hormone and fetal proteins, I cant remember the exact names of them I'd have to look at my results to tell you, perhaps someone here can remind me - PAP something (not sure). They wont tell you if there is definately an issue, they'll give you a ratio of the risk. Mine came back as 1:9500 but it varies from person to person and depends on a variety of factors such as age the NT fold measurement on the scan.
Someone else here will probably know more than me. but as others have said you'll get a call within a few days after the blood was taken if there was potentially a problem as they usually re-test.
xx