I went through that process.
They even interviewed my friends ( have no idea how they got their names!). As far as I can tell from memory, the form hasn't changed much.
The classification is hierarchical: Confidential, Secret, Top Secret, SCI (Special Compartmented Information). It took about six months to get my clearances. And then they told me I couldn't smoke any more dope.
I didn't learn anything that didn't show up in the press, eventually. As Mr Spock once put it (when they stole the Romulan Cloaking Device); "Military secrets are the most fleeting of all."