Relational Database Management Systems are probably the ones that we are most familiar with in 21st century computer science. Some of the history behind the creation of these databases is quite interesting. It started with an unknown researcher at IBM's San Jose, CA, research facility--a gentleman called Edgar Codd. Mr. Codd was working at IBM on hard disk research projects, but was increasingly drawn into the navigational database management systems world that would be using these hard disks. Mr. Codd became increasingly frustrated with these systems, mostly with their lack of an intuitive query interface.
Essentially, you could store data in a network/hierarchy, but how would you ever get it back out?