How many types of indexes or keys are in MySQL?

I want to optimize my database. So I need to learn about INDEXES or KEYS in MySQL. Can you explain that 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.

Primary Key Indexes

Unique Key Indexes

Normal Indexes also known as "Non-Unique Indexes", "Ordinary Indexes", or "Indexes without constraints"

Full-Text Indexes