27.5″ = 650b in terms of people talking about both, but they are really the same things.
29er wheels are now the big thing, that wasn’t quite as big as the evil manufacturers were hoping, so they compromised with 27.5 and unleashed that on everyone, telling everyone it was the best bits of 29 and 26 in one handy package.
narrow/wide are the chainring designs used on single ring set-ups to avoid the need for a chain guide. The narrow/wide refers to the teeth on the chainrings alternating in width and this coupled with a clutch rear mech (keeps the chain taut) is supposed to stop the chain coming off the single front ring.
Axle standards – erm, yeah. I’m still on QRs so have just ignored it completely.
Don’t forget about bottom brackets…..hollowtech II is still alive and kickng, but the evil manufacturers have also got on the press-fit bandwagon and you can now get BB-30, BB-91, BB-92 BB-other stuff.
People like Hope make adaptors so if you buy a bike with a pressfit BB and don’t like it you can stick an adaptor in and use a hollowtech II BB.
Probably best not to worry about it all too much and just get what you like. It’ll all be different in 2016 anyway.