There are public vehicles with set routes called "Carro Publico" or Publico. They travel from town to town so fairly short hauls. They have designated routes, ending in the center of towns, typically you wait along the route and they will see you and stop to pick you up. These public cars are vans, holding 8 to 12 people, don't expect a working AC, be ready to sweat. Since I am from the east side of the island I will give you some samples from there.
In the center by the plaza in Junco you will find a public that travels from Juncos to Las Piedras, others to Humacao, others to Naguabo. they are starting at the Plaza usually.
Metro areas have big busses like you see in US metro areas. Also there is a train in the metro area but I never used it or know the route as it was build after I left.
You pay cash for the Publico, not sure what the fares are now day but likely 5 or so bucks.
Check with the locals as to where the publics route are, the destination is usually posted in a sign on the windshield. If you are going away, you may need to use two or more publicos to get to your destination since they only travel short hauls.
There is always tax. Also a neighbor may give you a ride if they are free and you pay the gas.
One more item .... this will probably turn you off, Kids sometimes are crying in the public car, also they sometimes get sick, so you need patience and a strong stomach.
This link may be helpful, not sure how complete it is, it is probably not complete, but likely they can give you other phone numbers. Hope your Spanish is up to par, not sure if they speak English.
Some contact info:
http://www.mydestination.com/puertorico … ublic-carsAbout public cars:
http://www.legendsofpr.com/publiccars.htmMore:
http://gopuertorico.about.com/od/therus … blicos.htmI have not used a public in years, but like the price quoted of 20 bucks is for some that do much longer hauls, obviously a rental car or your own car is a lot more convinient.