[Deukhoofd]

It's more commonly known as a masonry layout, popularized by Pinterest. It's similar to a mosaic layout, but with a tighter fit for different height images. If one image is 200 pixels high, and another is 100 pixels high, you can draw them next to each other, and put another image in the remaining 100 pixels of the right column.

[andybak]

Actually masonry was I was thinking of. Where do this waterfall business come from then?

[Deukhoofd]

Same thing, different name. It's kind of like a waterfall in that it's a continuous vertical layout.

[aldousd666]

I was equally confused by this. I tried everywhere to find what the heck that even meant in this context and discovered that it is apparently a term they invented themselves.