Describe briefly the five database generations

First generation (50's and 60's): 

Files on tapes or cards.
The file systems used batch sequential processing only.

Second generation (60's): 

Files on disks.
The file systems allowed multiple direct access to random locations of a disk.

Third generation (mid 60's and 70's): Early database systems:

Systems started to separate logical and physical data structures.
Systems started to use implementation oriented data models, mainly including the hierarchical
model and the network model.

Fourth generation (80's): Relational systems:

Systems provided powerful, set-oriented query languages

Fifth generation (90's and 00's): Post-relational systems:

Designers are developing new systems that have enhanced functionality, more complex data,
and can serve a broader class of applications.