Data warehousing is a big subject. This overview is intended to cover some of the most representative issues on a high level: the nature of OLAP systems, star schemas, facts and dimensions, and differing perspectives (Inmon vs. Kimball) on warehouse design.
OLTP vs. OLAP
OLTP systems are the operational databases supporting applications. They are highly normalized, and focused on CRUD operations.
OLAP databases are usually arranged in star schemas and are built for speed in retrieving aggregated data.