Apache Spark is a fast, general engine for large-scale data processing. It provides unified analytics engine for batch, interactive, and stream processing using an in-memory abstraction called resilient distributed datasets (RDDs). Spark's speed comes from its ability to run computations directly on data stored in cluster memory and optimize performance through caching. It also integrates well with other big data technologies like HDFS, Hive, and HBase. Many large companies are using Spark for its speed, ease of use, and support for multiple workloads and languages.