There are two ways we can categorise the Databases. One can be termed as a RDBMS( Relational Database Management Systems) which was very popular and a structured Database This means all the datas are available in the form of Tables which consists of Rows and Columns. Each and Every table will contain a Primary Key to avoid Duplication. Apart from that the Primary Key of one table may be a Foriegn Key to another table. This can be termed as Referential Integrity. The other category which is now becoming popular is NOSQL Databases Which doesn’t fall under RDBMS. Here the datas are not stored in Table row format like the Previous one described. Coming to the specifics of your questions Some of the RDBMS Databases are SQL Server, Oracle,MYSQL, IBMDB2, IBM Informix, SAP Sybase, Teradata etc.And Some of the NOSql Database are Amazon S3 Dynamo, Mongo DB,Cassandra,HBase, Couch DB, Neo4J etc.