There's a lot of misinformation here, so let's trying to clear this up.
It all starts with a background investigation. Depending on the type of investigation done (e.g. SSBI) and the result, you may be granted a security clearance (e.g. Top Secret) that gives you access to collateral information. Typically, for the clearance to be active, you must be actively assigned to a position or contract that requires that clearance. That means that even if you have an SSBI and you've worked in a TS position before, if you're current contract only requires a Secret, then your active clearance is a Secret. If you change contracts to one that requires a TS, you can easily change as long as your investigation is current.
So let's talk about clearance statuses. Active means you're actively in a position requiring that clearance. If you're clearance is inactive, it means that you are NOT working in a position requiring a security clearance. Simple. Contrary to popular belief, clearances don't expire. The DoD and other agencies have guidelines for how often they like periodic reinvestigations (PRs) to be performed, but they are just that: guidelines. The standard for TS clearance PRs is 6 years, but that doesn't mean that someone whose last PR was 7 years ago can't get read onto a contract requiring TS: it's up to the adjudicating agency to make that determination.
A clearance revocation is an administrative action against a person. Your clearance doesn't get revoked because you left your job, your clearance gets revoked because you mishandled classified info, you lied on your SF-86, etc. What's not clear from the media - because the media is fucking retarded - is whether Brennan's clearance was revoked, suspended, or it was simply deactivated. If it was REVOKED, that means that someone made that determination that Brennan was not fit to hold a clearance and he is barred from holding a clearance, period. There is a process to protest a revocation, and if you win your clearance is reinstated. A clearance suspension is the step before revocation while they are looking into the situation.
Intel agencies can adjudicate based on your OPM (about to change to a new agency, FWIW) investigation. Here's where it gets less clear, because different agencies have different requirements, and those requirements can change depending on what SAP or compartments you're getting read into. Maybe the agency didn't care if your wife is a dual citizen when you're in one position, but they might if you're in another. Maybe one agency didn't care that your best friend was born in China and moved to the U.S. at age 3, but it's a dealbreaker for another agency. Intel agencies can also kick off another investigation if they feel it's necessary.
There are programs out there that only a handful of people - and sometimes less - are read into. Sometimes it is necessary to engage people who are no longer actively working on the program. That is why I assume that a lot of people who formerly worked in high-level positions continue to carry clearances. That's a guess - I've never been in that specific position, so I can't say for sure.
As for Brennan, fuck him.