Serial vs int schema data type options
Serial is used for auto-increment column of the table(so here auto increment is implicit), e.g. for node table nid has a serial data type and int can be any number based on size provided.


  'type': The generic datatype: 'varchar', 'char', 'int', 'serial',
  'float', 'numeric', 'text', 'blob' or 'datetime'. The types map to the
  underlying database engine specific datatypes. Use 'serial' for auto
  incrementing fields.


Reference: https://www.drupal.org/docs/7/api/schema-api/schema-reference