Trailhead is an interactive learning path through the basic building blocks of the Salesforce1 Platform. https://developer.salesforce.com/trailhead
Trailhead consists of three basic parts
Trails are virtual paths that help a user learn a certain concept or technology by aggregating different modules.
Each module covers one topic, and it's made up of one or more units that both teach and evaluate a subject. A Module can be present in more than one Trail.
By completing Trails and Modules one earns badges (or Superbadges, in certain cases) that help showcase the user's knowledge.