Hi Rin if you want a lens for wildlife why do you want a zoom. Nothing wrong with the sigma but you will get better shots with a prime lens, and you will probably use the sigma at the 400/500 end mostly.
Anyway to try and answer your questions, HSM = Hyper sonic motor sigma's version of canon's USM. DG I believe is a coating on the lens for digital not sure what difference it makes perhaps someone else will enlighten us both.
Zoom lenses automatically change the aperture as you zoom in and out, you will get a maximum aperture at what ever you are zoomed to and below this the lens will not go. For instance the sigma 500 @ 500mm is F6.3 (I think) and will not go below this but you can go up say to F8.
A nice lens to consider although I believe they are becoming scarce is the Canon 400 F5.6 or maybe the 300 F4 with converter. Others may have further suggestions but at the end of the day itis down to a) what suits you and b) how deep your pockets are.
Hope this is of some help.
Pete (also an amateur)
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