Security clearances

Confidential: The simplest security clearance to get. This level typically requires a few weeks to a few months of investigation.
Secret: A Secret clearance, also known as Ordinary Secret, requires a few months to a year to fully investigate depending on the individual's activities.
Top Secret: Top Secret is a more stringent clearance. A Top Secret, or "TS", clearance is often given as the result of a Single Scope Background Investigation, or SSBI. Top Secret clearances generally afford one access to data that affects national security,

List of the more popular and possible sets of security  clearance is below:

Confidential
Secret (Collateral)
Top Secret (Collateral)
DOE - C, L, Q
FBI (equivalent to TS Collateral)
NATO Secret
Public Trust
Yankee White
Top Secret/SCI
DOD Clearance
Full scope polygraphy