A relational database is a way of storing information (data). The database matches data by using identical information that is found within the data set. The resulting groups of data are organized and are much easier for many people to understand.
For example, a database has all information about real-estate sales in a town. This information can be grouped by year, or by sale price, or by buyer's last name and so on. Such a grouping uses the relational model (a technical term for this is schema). Hence, such a database is called a "relational database."
The software used to do this grouping is called a relational database management system (RDBMS). The term "relational database" often refers to this type of software.
Relational databases are currently the main way in which companies store information like financial records, medical records, personal information and manufacturing and logistical data.