The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
To put things in perspective:
Normal Hulk has one-shotted Carol every time they fight (2 IICR).
WWH has beaten Sentry.
Devil Hulk also casually one-shotted Thor (IICR a stronger than normal version).
Hulk has great showings against Hercules.
And Kluh, Hulk's Hulk KO'd Wanda just by telling her to "try harder" (kinda irrelevant I know).
Also, see the scan @Dhruv presented below about how Devil Hulk stomps WWH, and @Tyrannus' comments on how he would stomp Savage Hulk and Joe Fixit/Grey Hulk.