How many types of indexes or keys are in MySQL?

MySQL allows four general types of indexes (keys). These indexes can be created either on single column or multi-columns however both single column index and multi columns index have some different behaviors.

1. Primary Key Indexes
2. Unique Key Indexes
3. Normal Indexes also known as "Non-Unique Indexes", "Ordinary Indexes", or "Indexes without constraints"
4. Full-Text Indexes