Alla prima, or wet on wet, refers to a direct style of painting where paint is applied without letting earlier layers dry. Artists using this tecnique can finish paintings often in as little as a single session.
This technique differs from the more traditional method of painting layer on layer, allowing the paint to dry in between each layer. Artists would often build up many layers (over 50) before deeming the painting complete. Obviously this was a very time consuming method of painting, as oil paint can take anywhere from a few days to some months before being completely dry.
Alla prima painting was practiced by many of the impressionist masters, as it allowed them to capture the ellusive light as quickly as possible.
When painting alla prima using oil paints, it is important to follow the fat over lean rule to avoid cracks forming in the painting. This simply means you should more oil present in the later (the top) layers of paint. You can balance how fat your oil paint is using a paint thinner and an oil medium.