ACID stands for Atomicity, Consistency, Isolation, Durability. These are the key properties of a database transaction to guarantee correctness even (unexpected) failure occurs.
A set of columns that uniquely identifies a row in a table. It could be a single column such as UUID or a set of columns. Many database systems requires user to specify the primary key in the table schema, for those not specified, an internal primary key is always created since the primary key is required to identify and address a particular row.