Today there are consistent lists of calls that are in world-wide use. The lists are cumulative with the first six (Mainstream, Plus, Advanced, C1, C2 and C3A) being maintained by Callerlab. Each list contains all the previous lists. A dancer who has learned everything on the Mainstream list can be confident that they will know all the calls used at a Mainstream dance anywhere in the world. This standardization has made the wealth of square dance conventions, fly-ins, and festivals possible.
The difference between the levels is mostly the number of calls. At any square dance, without regard to level, you'll see folks moving together to the beat of the music, the dancers in each square working as a team to solve the puzzles the caller provides. At a typical Mainstream dance the emphasis is very strongly on the dancing with relatively simple puzzles to solve. For folks who enjoy more intricate puzzles the higher levels provide them while maintaining the dancing and teamwork.
There's a great video of a tip with sequences from every level. The folks in the video are some of the most enjoyable dancers in the world. Notice that they mostly move to the beat, and work well as a team. The dancing's not perfect, but everybody's enjoying it. That's what fun square dancing looks like, independent of level.