Final answer:
An RDD is a fundamental data structure in Spark for distributed data processing, allowing for parallel processing of data.
Step-by-step explanation:
An RDD, or Resilient Distributed Dataset, is a fundamental data structure in Spark for distributed data processing. It is not a data storage format, programming language, or type of database. RDDs are fault-tolerant collections of objects partitioned across multiple machines, allowing for parallel processing of data.