Adding to what @PratheepArunRaj said, the Hit based segments will only pull the results for those hits where the User ID value is not equal to a specific value which you have mentioned. However, a Visitor based segment will pull the entire data set where the User ID is not equal to the value you have mentioned. I will try to clarify with the help of an example. So, let's say we have a visitor with the User ID = 'abc1243' who have made multiple visits to your web site. So, if you use a hit based segment to filter the results for this user then only the hits where the User ID was set to 'abc123' will be returned. However, if if you use a Visitor based segment then all the hits from this visitor will be returned i.e. where User ID is equal to 'abc123' as well as the hits where User ID is not equal to 'abc123' if one of the hits matches your segment condition.