While they both use SQL, Amazon RDS and Redshift are two very different things.
Amazon RDS is a traditional SQL database hosted on AWS. It supports IBM Db2, MariaDB, Microsoft SQL Server, MySQL, Oracle Database, and PostgreSQL. You get high availability, backups, and so on.
Amazon Redshift is its own thing. It's a column-oriented database for Big Data, Data Warehousing, Business Intelligence, and Online Analytical Processing. It began as a fork of PostgreSQL 8 and has diverged significantly since.
Because it addresses very different requirements, the specialized data storage schema and query execution engine that Amazon Redshift uses are completely different from the PostgreSQL implementation.
Column-oriented databases are structured and optimized very differently than the row-based databases you're used to. There's no secondary indexes, no table partitioning, fewer constraints, no triggers, no sequences. Queries are automatically distributed to compute nodes.
See the Amazon Redshift SQL Reference and Amazon Redshift and PostgreSQL for details.