I strongly recommend this talk by Martin Fowler:
https://www.youtube.com/watch?v=qI_g07C_Q5I
ABSTRACT:
Martin gives a rapid introduction to NoSQL databases: where they came from, the nature of the data models they use, and the different way you have to think about consistency. From this he outlines what kinds of circumstances you should consider using them, why they will not make relational databases obsolete, and the important consequence of polyglot persistence.
It draws a nice picture of what NoSQL is, the different categories and the things everyone has to understand when coming from relational databases world.
Regards.
