Small but important difference. PL doesn't limit the points spent, it limits the effects gained. So a character that spends 50 points to have a power at rank 10 and a character that spends 4 points to have the same base effect (with very different extra effects) at rank 10 are both capped at rank 10 (the PL) not the 50 points.
If they were capped by points then second character could have something like 150 ranks in the power (ain't nobody making a DC 165 toughness save).
Yeah, you probably know that, but it's important when explaining it to others.
HERO has a few different limit knobs to play with (effect caps, point caps, or both) but by default doesn't use any of them.
Stacie, it might help to realize that the entirety of M&M is designed to be resolved with the mechanic "Roll 1d20, add stuff, and compare to a DC." Then, as a style thing, they made the game defensively oriented (it's not how well you got hit that matters, it's how well your resist that matters). All the other stuff is giving you abilities to attack with (powers), abilities to resist with (saves), "normal" abilities (attributes & skills), or abilities that connect the other categories (feats).
If that doesn't do it for you ... well, thanks for playing, have a good day.