Concert is not a standard radio (maybe on some markets only). Standard, very basic one is Chorus. Chorus does not have SD slot, nor AUX-IN and comes with monochrome screen in central console. Concert is a bit more advanced (color screen, traffic messages, RDS) and does have single SD slot (up to 32 GB) and have AUX-In (if you have central armrest it is hidden under, not sure where it is installed if there is standard console without arm rest). So in theory you can play iPod on Concert via AUX-in.
Chorus is the basic (Not available in the UK)
Concert is the next step up with SD / RDS / AUX / Colour screen - standard in the UK
Symphony is an upgrade in the UK (Standard on SE models) comes with same features as Concert but is 180W & 6Ch Amp
B&O = Ultimate upgrade for all models (Standard on S-Line Special Edition) 500W & 10Ch Amp
Function wise Symphony is almost the same as Concert (and also 80W), but has build in 6-CD changer.
180W is Audi Sound System, that is not related to radio you use - it is optional extra you can order to any radio or MMI (but not together with B&O). It contains of 6 channels amplifier and 10 speakers (additional 2 vs standard are mounted, I think, in the dashboard near pillar A, similar to B&O). Additional 2 channels are routed to subwoofer (installed inside spare wheel under the floor of trunk) and center speaker (installed inside dashboard, near the windshield).