What is the difference between senators and members of parliament?

"Parliament" is the Word used to describe the whole of the national representative body or bodies having supreme legislative power. In the USA, the collective Word is "Congress". "Senate" is almost universally used to describe only a part of that whole representative body.

In most countries whose Parliaments have a two-Chamber system (as in the USA) "Senate" is usually a sort of "upper House" that receives and reviews the laws already approved by the 'lower' House (in the USA: the House of Representatives) and that still can vote those laws down.

So, Parliament is the whole and Senate is a part of that whole.