A flush casement window has the opening part sitting completely flat within the frame, so everything lines up neatly with no parts sticking out, giving it a clean, modern look.

A standard casement window, on the other hand, has the opening section slightly overlapping the frame, so it sticks out a bit and creates a more traditional, noticeable shape.

The easiest way to think about it is: flush is flat and smooth, while standard stands out.