Droplets are one of the most important out of the box components provided by ATG. In reality a Droplet is a Servlet, Droplets are mainly used to abstract the business logic in server side code. Droplet helps the developer to separate the presentation code from the business logic code. There are around 250 out of the box droplets available in ATG 10. You may write your own droplet if the provided droplets are not meeting your requirements.
How to write ATG Custom Droplet?
Refer this post to write a custom droplet.