There are several levels of classified documents, and 5 minutes on wikipedia would tell you everything you'd ever want to know.
Protected A / B / C is stuff that's going to fuck up one person. So if we found out that, say, HC was actually a dude or had HIV or something, that would fuck up her life but America would not be damaged.
Secret / Top Secret (usually called level II or level III) is stuff that's damaging to the country. So III might be something like today's frequencies or the inner workings of the IFF system. II would be layouts for circuit boards in warship computers, etc.
On top of that you have Controlled Goods and ITAR stuff, but that's not really Classified.