Cadets is a weekly Sunday event where master raiders Silver & Cara train people with the commonly used tactics for all raids currently available in game. These are the hard mode tactics that are usually used for guild raids, and will stand you in good stead for most pugging opportunities, and give you a solid basis for a raiding career.
Make sure to read Silver's full post about the Cadets as it explains everything you need to know:
The team is making great progress so far. We are testing both the rollback and the Ossus fix as I type this. Assuming both of these pass testing, we aren't terribly far from coming online. I will pass on more info as I have it.