What is it
From Cloud Bigtable's website:
A fully managed, scalable NoSQL database service for large analytical and operational workloads with up to 99.999% availability.
When to use it
Assess Cloud Bigtable for use cases such as (but not limited to) high-throughput write workloads (e.g. timeseries), or big matrix-like datastructures.
Start with a one-node instance and scale up from there.
How to learn it
This paper provides an excellent introduction to both the high level concept of column-oriented databases, and specifics about the design of Bigtable.