The lower levels of information classification in use by the US government are actually a grid, contrary to popular belief. Along one axis are:
Material with no sensitivity
For Official Use Only (FOUO)
Formerly Sensitive But Unclassified (SBU).
Formerly Confidential.
Used to denote information which should not be released to the public, but whose nature does not adversely affect national security. There are no codeword levels to FOUO.
Secret (S)
Information whose release could cause harm to national security.
Top Secret (TS)
Information whose release could cause grave harm to national security.
The second axis contains:
A catchall for information classified at this level by its very nature.
Sensitive Compartmented Information (SCI).
Commonly referred to as "codeword". Information with an SCI designation will have a codeword designating the purpose of the classification. These codewords are classified at the same level (Secret/Top Secret), but Collateral.
SCI codewords are usually buckets for different types of intelligence sources; information gathered via humint might fall into one bucket, whereas satellite imagery would fall into another. What Saddam Hussein eats for breakfast would likely be classified TS/SCI; the information has no intrinsic value; the fact that the US knows it reveals information about an intelligence source.
There is secret and top secret data at the SCI level; it is not actually above those levels, but carries stricter requirements.
Special Access Programs (SAP)
This are programs which are only allowed to have a limited number of people aware of them at any one time. SAP members require third-party introductions to verify identity and clearance level. (A knows B, B knows C, A & C know of each other. For A&C to talk about a mutual SAP program requires an in-person introduction by B).

Given the shared nature of allied intelligence, much classified information will also designate a limitation on which countries can see the data. Typical subsets:

Things not covered here:

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