Melissa Coates has a nice introduction to columnstore indexes:
Many of the in-memory features in the Microsoft platform rely on the xVelocity (formerly known as VertiPaq) engine. The implementations do differ somewhat between products, such as the requirements for data to be truly memory-resident.
The remainder of this post will focus on the two columnstore technologies in SQL Server: clustered and nonclustered.
If you’re not very familiar with columnstore and aren’t quite ready to tackle Niko Neugebauer’s columnstore series, this is a good way to get started.