[SoleSky]

I am slightly confused on oxidation numbers.  What do they determine?
Also, when given a problem like this:
H2Cr2O7, how do I know which element I'm supposed to find the charge to? If you could please help, or if you need me to reword the question I will, thanks.

[Borek]

I am slightly confused on oxidation numbers.  What do they determine?

Nothing. They are just a device to help you in electrons accounting, but there is no measurable property of atoms that can be linked to oxidation number.

Also, when given a problem like this:
H2Cr2O7, how do I know which element I'm supposed to find the charge to? If you could please help, or if you need me to reword the question I will, thanks.

H and O are almost always +1 and -2, so only Cr is unknown here.