Help....regarding service tax

hey,

During my audit i came across 4 A/c in my client's Tally system i.e

1. Service tax (paid/credit) A/c

2.Service tax payable Expense A/c

3.Service tax payable A/c

4.Service tax A/c

Can any1 plz explain to me what is recorded in each of these accounts,

also mention the way of accounting the same.

And when (which cases) should service tax be reversed ? 

Thank you