Wide band: 5KHz deviation, uses a 25KHz wide channel.
Narrow band: 2.5KHz deviation, uses a 12.5KHz wide channel.
As you mentioned, as of January 01, 2013, most part 90 users in the VHF and UHF bands need to be narrow band. Low band VHF is not affected, neither is 800MHz, Ham radio, GMRS, CB, Marine VHF. FRS and MURS are already narrow band, and have been since their respective beginnings. There are a few paging frequencies that are allowed to remain wide, but that's it.
Most newer amateur radios out there, at least the ones I've seen in the last few years, are narrow band capable. The more recent Yaesu and Kenwood radios have been. Ones I know of for sure:
Yaesu FT-7800 and 7900
Yaesu FT-1820
Yaesu VX-170
Kenwood TM-D710
Pretty sure my Icom IC-2200 was, also.
All new LMR radios have been required to be narrow band capable for quite a while now. Since a number of the amateur radios are based off commercial rigs, they have been following along.
The reason behind forcing users to Narrow band is that you can fit more users into the limited amount of spectrum available. It's been in the works for a long time now.
FCC is already hinting that there will be another "narrow banding" likely coming in down the road. Going from 12.5KHz to 6.25 currently requires going digital to get enough space to get useable human voices. You don't need to worry about that now.