Colombian cities, towns and villages have traditionally been laid out on a grid plan. The streets running north–south are called Carreras, often abbreviated on maps to Cra, Cr or K, whereas those running east–west are called Calles, often labeled on local maps as Cll, Cl or C. This simple pattern may be complicated by diagonal streets, called either Diagonales (more east–west and thus like Calles), or Transversales (more like Carreras).
All streets are numbered and the numerical system of addresses is used. Each address consists of a series of numbers, eg Calle 6 No 12-35 (which means that it's the building on Calle 6, 35m from the corner of Carrera 12 toward Carrera 13), or Carrera 11A No 7-17 (the house on Carrera 11A, 17m from the corner of Calle 7 toward Calle 8).
The system is very practical and you will soon become familiar with it. It is usually easy to find an address. It's actually one of the most precise address systems in the world; if you have an address you can determine the location of the place with pinpoint accuracy.
In the larger cities the main streets are called Avenidas or Autopistas. They each have their own names and numbers, but are commonly known just by their numbers.
Cartagena's old town is the only Colombian city where centuries-old street names have withstood the modern numbering system. Streets in some other cities (eg Medellín) have both names and numbers, but elsewhere only numbers are used.