Of course it depends on the field, and for top C-level executives the bonus is pretty much the biggest chunk of compensation. My particular bonus rests on my performance, and is based on profit besides, which is the most common form of bonus I've seen in the trenches of software development.
And they may not be treated the same, legally. In Massachusetts, salary is sacrosanct; it is illegal to have an employee working for you if you don't currently have enough money to pay them. But bonuses () are wide open. You can decide not to pay somebody their bonus for any reason, even if the appropriate manager has already said, yes, you've earned it this year. (Yes, this happened to me.)
() Discretionary bonuses, that is, where a human decides what you get, rather than a fixed rule.
