Surfing on the web, you can find tons of tutorial on how to build your own drone. DIY Drones is obviously a reference point. Here you can find some links to short but effective tutorials which explains you the basic technologies which are used under the hood in these platforms.
- Anatomy of a multirotor drone
- Aerial Mapping Drone for Under $60? (credits for the images in this post)
- Quick Drone Components Overview Along With Handy Tips