As noted in preceding sections, there are two types of operation that can be used to shape data in an RDD: transformations and actions. A transformation, as the name suggests, transforms one RDD into another. In other words, it takes an existing RDD and transforms it into one or more output RDDs. In the preceding recipes, we had used a map() function, which is an example of a transformation to split the data by its tab-delimiter.
Transformations are lazy (unlike actions). They only get executed when an action is called on an RDD. For example, calling the count() function is an action; more information is available in the following section on actions.
