Parquet files and Delta Lake

Column-based format - files are organized by column, rather than by row, which saves storage space and speeds up analytics queries.

Apache Parquet is implemented using the record-shredding and assembly algorithm, which accommodates the complex data structures that can be used to store the data. Parquet is optimized to work with complex data in bulk and features different ways for efficient data compression and encoding types.

Parquet and Delta Lake

The open source Delta Lake project builds upon and extends the Parquet format, adding additional functionality like ACID transactions on cloud object storage, time travel, schema evolution, and simple DML commands (CREATE/UPDATE/INSERT/DELETE/MERGE). Delta Lake implements many of these important features through the use of an ordered transaction log that makes data warehousing functionality possible on cloud object storage.

source: https://www.databricks.com/blog/what-is-parquet