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People use the two interchangeably and it's pretty annoying.
Imagine you're uncomfortable with your partner going out with people of the opposite sex (I don't care what y'all do; some people are ok with this, some aren't. You do you.)
A rule is "I don't want you doing x", or "I don't want you going out".
A boundary is "If you do x I will y" or "If you go out with that person I will x."
One is trying to tell the other person what to do. You can encourage or ask them to do things. Hell, the best part is when you two discuss something and come to an agreement.
But, you're still wanting them to change their behavior. You don't have control over them. You have control over you.
That's important, because all you can control at the end of the day is you.
I'll see people tolerate disrespect, or sit in relationships that don't serve them, trying to set "boundaries" when it's just setting rules. Of course you're gonna be frustrated, you put all the control in the other person's hands.
If someone's treating you in a way, do something about it. That's setting boundaries.