Hi everyone, I can't seem to figure this one out...

There's a LOT of different codes / types of lens out there. I'm looking at buying something which will go beyond my kit 28-55 VR lens that came with my nikon D3100, to do the more long-distance bird / wildlife shots.

First up, what would be the drawbacks of a lens such as this one from our sponsors:
http://www.qualitycamera.com.au/niko...6g-p-4584.html

First up, I realise the F4-5.6 is fairly "general" and this really is all I'm after at this stage, for the price it seems like an absolute steal, I just want to make sure I'm not missing any deal-breaking info here.

Is it "DX compatible" (what IS DX?)

Is it AF-S Compatible? do I need to manually focus this lens on my D3100?

Is there a good reason why I should avoid this lens (as a beginners lens)

And HOW can a noob like myself tell if a lens such as this one:

http://www.qualitycamera.com.au/tamr...ro-p-4244.html

... will be useable/compatible with the D3100 body that I have? I would hate to order something like this and then go to use it and my SLR say "incompatible lens" on the screen or something like that!

Would the tamron lens work with the autofocus on the nikon? would the apeture controls work?

Sorry if I seem a bit paranoid about this - the guy in the camera shop I visited last told me that "the one big thing with a nikon camera is that you need to buy the right lens for it, or else....." well, he didn't exactly say what, but I'm guessing it's bad! lol!

Thanks anyone who can shed some light on this for me
I'd LOVE to grab one of these lenses if they're not too "cheap'n'nasty"

-Jazz