Medellin Metro Plus Finally to begin Operation:
A new system of exclusive bus lanes are in place in some parts of the city and are supposed to begin operation in March 2010. Generally speaking El Centro (downtown), is just a huge bus terminal. People which go to work and use the buses generally have to change buses in El Centro (no transfer sytem on private buses). There are green bus lines that feed the metro, and you can get transfers with the green buses and the metro line.

Buses to Pueblos near Medellin:
Micro Buses I recommend using the small buses to get out of the major cities to visit the pueblos if you dont know the way. You can get to nearby villages for anywhere from 3,000 pesos to 6,000 pesos or so. Express van services to Santa Fe Antiquia runs about 10,000 pesos each way, (about 5 US dollars).  Always ask around to be sure the area is safe before venturing out. Major tourist destinations have been secured by President Uribe, and are fine places to visit. Stay over night at popular pueblos. Usually the popular pueblos (villages) have a few hotels around the town square. Search the internet for hotels, or finca hotels, and you will be suprised. They are actually showing up on the internet now. 

Medellin Bus Terminals:
If you are leaving the city you will need to know which of the 2 Medellin bus terminals you need to depart from. Just remember that shortt distances on a Colombian Highway take much longer than in the USA. The highways are very secure today, although it's best not to travel at night time.