A flutter kick means keeping your legs fairly rigid at the knees and kicking up and down with your fins moving in an arc. The two legs travel in opposite directions and reverse, like walking. Most of the propulsion comes from the down kick, a little ankle action can be added to make the most of your fins. The key point is that your upper leg and buttocks is doing the work, not just from the knee down.
A dolphin kick is more of a boddy and leg undualting wave with the legs remaining together, side by side.
The frog kick takes a little more practice and can be very hard to get the hang of if you have the wrong fins. It has the advantages of efficiency, less silting and it leads to some secondary kicks that let you pivot and even swim in reverse.
The vast majority of divers start with the flutter and go no further.