Difference between "come here" and "come over here"?

The short answer is 'not much'. The two expressions are more or less interchangeable.

But it can depend on distance. 'Come over here' would be used if the person you were calling was some distance away, at the other end of a hall, let's say, and there were other people in between.

'Come over here' is also perhaps a little more polite. Either expression would be enhanced by adding 'please', but 'Come here please' can sound slightly imperious in certain situations, depending on the relationship of the individuals. A parent or teacher might say it to a child, but it is not the sort of thing one would perhaps say to a colleague, without adding something like 'Would you mind coming here a moment'.

'Come over here please' sounds a bit better than 'come here please'. There are other things you can add to 'come here' which makes it sound less stark e.g. 'Come here a minute' said in a tone as though you have something important to show them, would perhaps not require even a 'please'.

I'm sure a lot more could be said on the subject, but I hope that helps.