Droplets are ready to use pieces of PHP code for use with the Droplet module. A droplet can be used by placing the corresponding tag in the WYSIWYG editor or directly in templates, so you don't have to use the actual code. It's easy to add new droplets through the backend.
WB2.8 comes with the droplet module and a number of droplets (available in the Admin Tools, in the backend) already installed. WB2.7 requires manual installation of the Droplet module first.
Read all about it on http://www.websitebakers.com/pages/droplets/about-droplets.php
. Also on that site you'll find all available droplets, that you can add to your site. Snippets
Snippets are pieces of "raw" PHP code for use in the Code module, or in a template. Snippets can only be used as code, not as tags. So they are a bit more technical and less user friendly than droplets, but sometimes you can really achieve great things with a bit of extra snippet code.
Because snippets often are transformed into droplets, and they both are pieces of PHP code that extend WB, you can discuss them both in this forum.