I could go into a detailed technical explanation about direction and ting and things like that, but I feel like that would end up confusing you more, so what I’m going to do is give you the simplest way to identify cartwheels v. roundoffs, focusing on visuals.
To be very, very simplistic, a cartwheel lands one foot at a time, one foot in front of the other. Basically, the gymnast starts in a lunge position and finishes in that exact same lunge position, only facing the other way:
A roundoff, on the other hand, begins much like a cartwheel, but finishes with the gymnast snapping the legs together before landing and landing with both legs together at the same time. So instead of landing one foot at a time with one foot behind the other (cartwheel) the gymnast lands with both feet at the same time and aligned right next to one another:
Does that help?
P.S. Sorry I had to answer your question by screenshotting your question and then uploading via photo post, but it was the only way I could fit two gifs into the answer field. For some reason Tumblr was being a dick and not letting be use a second gif when I tried to just answer using the inbox. Hope this suffices.