Contribution vs. novelty in the papers, especially in computer science?
It depends.  Novelty simply means "something that hasn't been done before", while "contribution" specifically talks about pushing the state of the art and solving a problem that hasn't previously been solved (or solving an old problem better).

To illustrate the point, assume you are working on a big data application and you realize that no one yet has ever tried algorithm X on that problem.  You apply algorithm X and try to publish simply because no one has done so before.  This is novelty, but it probably wouldn't be considered a contribution unless algorithm X is clearly better and you provide a solid justification for why.

A contribution is all about pushing the boundaries of what is known.  To continue our example, let's say now that you produced a new algorithm, Y, and are able to prove that in this specific problem it is better compared to other algorithms.  Then this would be both novel and a contribution.

When writing a paper with a "contributions" section, you usually should stick to the contributions in the sense described above - how does this work push the boundaries of the field.  

It should be clear from the introduction that:


A problem exists in the field that hasn't yet been solved (or that can be solved better)
Your work solves/helps that problem (this is the novelty - if your work doesn't do this, then what's the point?)


Then in your contribution section, you can specifically explain what your contribution is ("we propose an algorithm for... that uses... to solve...")