what is the difference between mysql community server and mysql cluster I am starting to look into different database's for different use cases.  I see there is a mysql community server and mysql cluster.  What is the difference between the two?  Are they both database engines/servers or is one an additional service for the other?
MySQL cluster is a storage engine for MySQL, which distributes and replicates data in an in-memory cluster (NDB data nodes) and can be queried via MySQL servers (SQL nodes).

Where it gets confusing, is that MySQL Cluster also has a native API where you can avoid the MySQL storage engine system and communicate directly to the data nodes with the NDB API, which is kind of NoSQL-like.  I'm not aware of any other storage engines that have this ability, and there are many large customers preferring this method of usage.

MySQL Cluster also has it's own release cycle and versioning system, and is currently at version 7.3.