The "getting started" tutorials are written for developers who are new to Apostrophe. They will walk you through the process of getting your environment set up, creating a basic website, and even diving deeper into some of the more advanced features.
The "intermediate" tutorials are geared toward developers who are more comfortable with Apostrophe and node.js.
The HOWTOs cover miscellaneous topics like configuring the admin bar, adding a field to every type of piece at once, and running Apostrophe under Windows.
Feel free to skip around after you complete the getting started series. Just keep in mind the material does tend to build on previous tutorials.