Associate Professors are usually tenured. Assistant Professors usually aren't.
The untenured position is really a sessional instructor, which I'm sure also goes by different names, but involves zero job security and extremely shitty pay. It's not a salaried position, it's a "per course you teach that semester" position.
This is true for some fields, at some schools. Median salary for an associate professor (the first rank with tenure) at my institution is 63k.