Here is my understanding of cloudfoundry buildpacks and droplets
buildpack is runtimes(say, jdk) + containers(say, tomcat) + frameworks(say, spring) + service configs (if any) + your apps (if any).
droplet is buildpack deployed on a linux container.
Please correct or add additional information.
A buildpack is a collection of three binaries: detect, compile, and release. When your app bits are pushed to Cloud Foundry, the detect from all the available buildpacks are run against your bits until one returns true. For example, the Ruby buildpack checks to see if there's a Gemfile, the Python buildpack looks for a requirements.txt, etc. Next, the compile phase turns your app bits into a single runnable package, which means compiling the code (if necessary) and bundling in any additional things needed, such as JDK, tomcat, spring, etc; or a Ruby interpreter, gems, etc. That single executable is the droplet. Finally, the release phase presents the droplet and associated metadata so that the stager can upload them to the cloud controller.
To run your app, the droplet is retrieved from the cloud controller and run inside a container.
Basically, a buildpack packages your app into a droplet, which consists of your app + some other stuff, or a compiled binary version of your app, and the droplet is then run in a container.