?
?
It depends on the school and the department. If you go to the University of Minnesota, you won't even find a course with a three digit number! But at Harvard, the course called Math 55 is supposed to be one of the hardest undergrad math courses in the country. (It covers material from Harvard's Math 25, among much more stuff.)
I go to Wisconsin. In my school's computer science department, almost every course is at least 300 level... but I've taken the standard 300-level intro comp sci courses, and in my opinion there are harder 200-level courses from the history department.
And in the math department, in my opinion, you won't find a hard class until the 500 level at least. (The class that Harvard labels 122, we label 541.) And I've had 400 level courses which were easier than some 200 and 300 levels.
Assuming you are talking standard
100 intros
200 Specifics
300 Majors/Minors (for the most part)
400+ Majors only (For the most part)
It really depends on the class. 200s generally aren't that bad, require a prereq or two but that's about it. Just email the prof if you are concerned.