Heya! I was also confused a bit over the difference between gesture and rhythm, but the way I think about it is this: Contour: Just the outline, like if you were tracing an arm its only the exact lines along the outside (While it sounds like the best in theory because its the most accurate, it looks weird a lot of the time because it feels stiff. We use methods like gesture and rhythm as a way to help solve this, even if at first glance they both look like contours.) Gesture: A way to show movement that can come in many forms. It could be a single line that shows the direction an arm's going, or two lines to give a more 'accurate' arm but not too worried about the individual bumps. You could go further with more complicated lines like one curved one s-shaped to get even closer to the real shape of an arm but no matter what the focus is still on its big movement Rhythm: Extremely close to gesture because it still focuses on the broad, sweeping movement, but is now extra concerned with connections and flow. Going back to the arm, you could split up the arm into the upper arm, forearm, and hand and it would still show the 'gesture' or 'movement' but it would still feel really stiff and disjointed. To help with this you look for ways that all of the parts connect. This is where it gets difficult because it's different for every pose. In some poses, there's a nice 'rhythm' or 'flow' or one 'big, simple line' from the upper arm to the forearm into the hand, but other times the elbow is bent. Sometimes there's one really big, cool (sometimes invisible) line or 'rhythm' from the top of the head all the way to the toes, but most of the time it's not that easy. Finding a rhythm is really about finding the biggest possible connections in each pose in as many places on the pose as possible, but it's hard because a lot of the time it doesn't follow the 'outline' or 'contour' but goes 'through' it in places you can't see it. Hope this helps :)

Here's a couple more with refs and some level 2's at the end

Here's the references for these guys