Tulum to the Crucero. Those marked R2 or R15 go to Wal-Mart. All other busses are a complete mystery that will not be solved by vacationing gringos.
Service is quite frequent.
During the evening rush hours, crowded busses will sometimes pass without stopping, often because they are on special contract transporting workers for some hotel or construction company. Just try your best to understand the system, as we never quite got the gist of it in some two years of commuting from Puerto Morelos to Cancun. You buy your ticket at either of the two sets of windows representing the different lines. The difference between first and second class busses is not immediately distinguishable, except there is reserved seating on first class long distance routes. Get to the bus station as early as possible in the morning and make sure that you know what time the last bus in the return direction leaves.