count() simply counts the number of elements in the RDD and sends it to the Driver.

The following is an example of this function. We created an RDD from a Sequence of integers using SparkContext and parallelize function and then called count on the RDD to print the number of elements in the RDD.

scala> val rdd_one = sc.parallelize(Seq(1,2,3,4,5,6))
rdd_one: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[26] at parallelize at <console>:24

scala> rdd_one.count
res24: Long = 6

The following is an illustration of count(). The Driver asks each of the executor/task to count the number of elements in the partition being handled by the task and then adds up the counts from all the tasks together at the Driver level.