There's 2 big ways power level effects characters in M&M - total power points and power caps.
The power caps are the big issues if you start going below PL 8.
Basically - the PL number is the max offensive and defensive value your characters can have. You can swap between them if you want to. Example: a PL 8 character can have +8 to hit/ +8 damage, or +6 to hit/ +10 damage, or +4 to hit/ +12 damage, etc. That's just attacking, there are skill caps, defense caps all over the place. Gear is included in these caps. This is to stop somebody from getting a damage 10 anti tank gun in your PL 6 cop game and insta killing everyone.
SWAT officers in the main book are PL 6, and random thugs are PL 4. Heroes really ought to start at PL 8 and up so they have a better chance at survival vs random folks.
PL 8 - good for teen heroes (powered but inexperienced), and costumed vigilantes. Think the Teen Titans, Hit-Girl, Ninja Turtles
PL 10 - standard starting point assumed in the book. Established heroes. Think Spiderman, most of the X-Men, The Punisher, Daredevil, Catwoman.
PL 12 - As 10 but moreso. National level heroes and villains. I'd put most of the Avengers about here. Also folks like Apocalypse.
PL 15 and above - Justice League A-Lister or cosmic beings. Think Superman, Wonder Woman, Thor, Silver Surfer.
Outside of DC adventures I don't think they've ever released a list as exact as to point out specific character examples specifically. You could always try the M&M forums over at roninarmy.com for better ideas for power level scaling. There are some pretty detailed discussions on the matter there.