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:
If a moderator has warned you via email about behavior, and that behavior continues, for a period of 2 to 7 days, your account will be in timed suspension.
Your account will be locked at 1 reputation. Your user page will have a visual indication that you are in timed suspension, and for how long. You will be unable to ask or answer questions. At the end of this period, your reputation will be recalculated, and your account will resume as normal. As I said, we don’t hold grudges; the point of all this is to address the behavior. If the behavior improves, we’re cool.
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.