In clicking "No Action Needed" you make a judgement, and the system will use your judgement to help decide what action to take in respect of a post. It may be that your click, when combined with other those of other reviewers, tips the system into taking no action (removing the post from the review queue entirely).
By clicking "Skip" you are declining to make a judgement, so the system has no additional data and will need to carry on asking others. The post will remain in the queue until there is enough data to decide what to do. In addition, declining to make a judgement will not increment your review count for progress towards review badges: by skipping the post you haven't actually reviewed it.
In both cases, the system knows that it does not have to ask you again about that post.