I've been reading a lot about the oxidative dissimilation etc, and often I see different sources use NADH and NADH2 in the same reactions. One source uses NADH and another uses NADH2 in the exact same way. Is there a difference or is it just a different name for the same substance?
They are often used interchangeably to indicate the reduced form of NAD+. The overall reaction when oxidizing some molecule RH2 is: RH2 + NAD+ -> NADH + H+ + R. The proper reduced NAD+ is NADH (it accepts two electrons and one proton), but sometimes NADH2 is used to account for that second hydrogen that gets removed from the substrate being oxidized. The notation NADH2 doesn't really take into account the fact that the second hydrogen is charged, and not bound to the NAD in the same way that the first hydrogen is, so it is confusing. The notation: "NADH + H+" is more correct and is also sometimes used.