New to number theory (few questions)

[O_OO_OO_O ()]

1. How would you represent a fraction in set Zsub5, which is the set of all integers mod 5?
2. What are 'units' and what is the set Usub m?
3. What is the order of a unit o(u)?
4. What is Z[x], Zsub m[x] and Q[x]?

Thanks in advance! :))