Although this guide is looking at the differences between the D3100 and D3200 we must point out there's no difference between the D3100's and the D3200's AF system. Nikon has stuck with the 11-point Multi-CAM 1000 auto focus module on both DSLRs. Along with Single-point AF and dynamic-area AF, there's auto-area AF and Nikon's clever 3D tracking. This mode will track your subject from AF point to AF point as it moves across the frame.
Much like the D3100, the D3200 doesn't feature a built-in AF motor. This isn't a major issue, as the majority of the Nikon lens line-up features lenses with their own built-in motors (designated AF-S), but it is worth bearing in mind if you're planning on using or buying older designs that don't offer this, as manual focus will only be achievable.