AMD's Name Rebadging Help!

[Linnaeus]

I've very confused with AMD's rebadging. I don't know what the equivalents are of the R7/R9 series is to a HD 7000 series. Also, what is the point of rebadging? Are they completely different GPUs?

 

[sourodip]

R7 265=HD7850
R7 260x=HD7790
R7 250x=HD7770
R7 250=HD7750
R7 240=HD6670/HD7730
R9 270=HD7870
R9 270x=HD7870 XT
R9 280=HD7950
R9 280x=HD7970
R9 290/290x are new and are made to counter the gtx780/780ti and the 290x is somewhat equal to hd7990.
The new series consumes somewhat less power and are cheaper and nothing else.