The other thing to note is that I am going to be covering Jenkins 2.0; at the time of writing this, the first beta has just been released, which means that while things may change slightly as themes and such are refined, all of the features and basic functionality are locked in.
The reason for covering Jenkins 2.0 rather than the Jenkins 1.x branch is that as far as Jenkins is concerned, Docker is now a first-class citizen, meaning that it fully supports and embraces the Docker way of working. A full overview of the current status of Jenkins 2.0 can be found at https://jenkins.io/2.0/.
So what is Jenkins? Jenkins is an open source continuous integration tool written in Java, and it has a lot of uses.
Personally, I am really late to the Jenkins party; being from an operations background...