Amazon Redshift is a hosted data warehouse product, which is part of the larger cloud computing platform Amazon Web Services. It is built on top of technology from the massive parallel processing (MPP) data warehouse ParAccel by Actian. Redshift differs from Amazon's other hosted database offering, Amazon RDS, by being able to handle analytics workloads on large scale datasets stored by a column-oriented DBMS principle. To be able to handle large scale datasets Amazon is making use of massive parallel processing.
Amazon Redshift "is based on PostgreSQL 8.0.2. [...] PostgreSQL 9.x includes some features that are not supported in Amazon Redshift. In addition, there are important differences between Amazon Redshift SQL and PostgreSQL 8.0.2".