I'm looking to learn how to build something, but I have no idea what it's called, so I can't even do a proper search on Google.

Basically, it's two photos in the same space, and some kind of bar that you can slide left or right, to show more on the right (or left) side of the sliding bar. I've seen it used to show (for example) the same picture, one in B&W and one in colour.

Hope my explanation makes sense.

I'm looking for a simple tutorial, nothing fancy, I want to learn the basics first.

Can anybody point me to a beginner's tutorial (preferably not on YouTube, though I guess I'll have to have YouTube if there is nothing else in writing)?

Or even just tell me what search terms I could use to find what I'm looking for?

Thanks in advance!