What are the differences between composer update and composer install?

Here are few key differences:

Point of Difference Composer Update Composer Install
Update and Install latest Dependencies Yes. It read composer.json. Check for latest version of the package and install it. No. Install all dependencies specified in composer.lock file
What if no composer.lock file Create it run composer-update and create it
Update composer.lock? Yes No
When to use? Development Phase Deploying Phase