About

Iteration, often referred to as looping or batch processing, means to repeat a process over and over with some degree of automation. Iteration is very important because automating repetitive tasks reduces the time and effort required to perform the tasks.

With iteration a process can be executed over and over using different settings or data in each iteration.

http://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/vector.html#iterating-over-vector-layer