"BigTable" is Google's proprietary database technology used by several of their major applications including: Google Reader, Google Maps, Google Book Search, "Search History", Google Earth, Blogger.com, and YouTube.
It is also the database used by the Google Application Engine.
Google describes BigTable as "a sparse, distributed multi-dimensional sorted map" which can scale to hundreds of processors. It is not a traditional relational DBMS and intentionally does not support the "join" statement. It is similar in many ways (AFICT) to Amazon's SimpleDB.
There are some good articles and links in Wikipedia - search "BigTable" or "GQL".