What is the unit cell and coordinates for wurtzite crystal structure?

[aaaa202]

As far as I understand wurtzite is the name given to the crystal structure defined by the stacking sequence ABABABAB..
However, I am having a hard time understanding what the unit cell is of wurtzite. From what I know the unit cell is the smallest repetitive unit and in it you specify the position of the different atoms. So I looked up something about wurtzite and found that data:
http://som.web.cmu.edu/structures/S014-ZnO.html
And according to this you only need to specify the position of two atoms, while the unit cells seems to encapsulate a lot more atoms - why are the positions of these not specified?
And another question: The meaning of the coordinates (x,y,z) is x*a1,y*a2,z*a3, where a1,a2,a3 are the unit vectors of the hexagonal lattice - right?
Thanks for taking your time

 

[Alkim]

The answer is easy. Two atoms do not form a cell but what is called an asymmetric unit: http://en.wikipedia.org/wiki/Crystal_structure#Unit_cell
The unit cell is obtained from the asymmetric unit by applying symmetrical operations.