How do the payrole work?

[immitech]

I am going on w2 contract with employer. How payroll works?

these are details that I have.

billing percentage w/employer : 70-30%
relocation, Medical, SSN, fed and state tax paid by employee
what is employer tax? they say 10% from their 30%.
They hold my H1 and GC.

How do these(Med,SSN,Fed,State tax, ANY OTHER HIDDEN) numbers work when I charge say $50 on w2.

thanks in advance,
immitech