A relational database is a database that organizes data into one or more tables. A table is a collection of rows and columns. A column defines the name, size, format, and type of data stored for that column in each row. A row is a related set of values that adhere to the constraints of the columns.
Relational databases require that each row be uniquely identifiable by a key made up of one or more columns. Rows can refer to other rows in the same database by a foreign key to form relationships that are validated and enforced by the underlying database. The vast majority of relational databases support SQL for data selection and manipulation.
Deeper Knowledge on Relational Databases
Structured Query Language (SQL)
How to learn SQL: The language of relational data
Microsoft SQL Server (MSSQL)
Microsoft's flagship relational database
A popular open-source relational database
MySQL: An open-source relational database management system (RDBMS)
Broader Topics Related to Relational Databases
Data that strictly adheres to a schema
Organized collections of structured data