Bank is a 0 hours contract. You work the shifts you want to/can do, on the wards/departments you want to/those that have uncovered shifts. You get paid the hourly rate for that shift (e.g. if you're a band 5 covering a band 5 shift, you get paid bottom rate band 5 per hour to cover that shift).
People work on bank either as their primary contract or as additional to their substantive contract.
Overtime is agreed shifts on top of your contracted hours (for your own department). Generally gets paid time + 1/3.
So for your own ward, when covering an otherwise unfilled duty on top of your contracted hours, you can do it as a bank shift or ask for overtime. If that shift is on another ward/department, you can't get overtime as you're not contracted to work there.
Sometimes you'll get places offering bonuses on top of pay for doing bank shifts. E.g. over the winter, my place offers a £50 bonus for a bank half shift and £100 bonus for bank full shift, which then ends up paying more than doing overtime would...