Difference between 'indexes' and 'primary keys'?

[jake66]

What is the difference between indexes and primary keys in phpmyadmin? (What way should they be used to gain the most speed?)

I tried to search for an existing topic as I'm sure it already exists, but couldn't find anything.

[physics]

A primary key must be unique. Usually it's an integer ID that uniquely identifies each row.

Indexes are used to ... index ... the values in a given row so that they can be found more quickly.