This information is for people who don't live in Cancun, and speak little or no Spanish. First, the buses in Cancun are a cheap way to get around. They are perfect if you are staying in a hotel on the strip and want to go to the marina or the tourist downtown (not the actual downtown). People will tell you that you just tell the bus driver where you are going and he/she will tell you when to get off. Not true. They will tell you if they go where you want to go, but will not tell you when you arrive. The three private companies will tell you when you reach your destination, but it is difficult to tell which bus you are on.

 Once you are off "the strip" it becomes difficult. To get to the real downtown, the bus will let you off, but figuring out how to get there is difficult. The easiest way is to then take a taxi, which will cost you about 35 pesos. When going back to the strip, don't walk to far. If you walk too far (less than a mile) you get to an area that when heading east (back to strip) that has no bus stops for 2-3 miles, but you will find plenty of Iguanas and "Caution Crocodiles" signs. 

 So, for travelers, use the buses, but stay on the strip. For farther trips, use taxis.