What are the differences between account suspension and ban from stackoverflow (and similar sites)?
Why would someone get banned? I mean what are the criteria for the ban and what should someone do to uplift the ban?
There is no difference between the two. The whole process of suspension is explained on the blog here. The most important part:
Effectively the only thing that would change is the length of the suspension. However, a lot of effort is made to give a user the opportunity to change before longer suspensions are enforced.