Global Entry is a service for US citizens that allows you to bypass normal Immigration lines when arriving back into the USA.  Here's how it works:

- You apply online and apply through their website (beware: it is a government website, so it is not the easiest site to use and navigate).

- Pay the $100 non-refundable application fee.

- They (not exactly sure who 'they' is) do a background check on you.

- Once your application is approved, you need to schedule an interview at one of the Global Entry Enrollment Centers.

- At the Center, you get interviewed, they take your photo, and collect biometrics from you (fingerprints, and retina scans).

- Once you are approved, you will save LOADS of time.

As a Global Entry member, I can literally fly through the airport.  Whenever I arrive from an international flight, I bypass the normal Immigration lines, and head straight towards the Global Entry kiosk.  This is what it looks like:

You scan your passport on the machine, and it reads your information.  It displays your name and your Arriving Flight details, then asks you to place your fingers on the fingerprint reader.  Once confirmed, it will ask you a series of questions (the same questions that are on the little blue USA Customs form that they hand out on the plane).  Answer the questions on the touchscreen, confirm, and it prints out a 'receipt'.  Hold up the receipt to the nearest Immigration officer as you walk by towards baggage claim.  Once you have your bags, get in the Global Entry express lane, hand your receipt to the Customs agent, and you're done.

You don't have to fill out the blue Customs form on the plane. You don't have to wait in line and see an Immigration officer.  And, you get to use an express lane to depart through Customs.  For me, this was a no-brainer, and it has helped me get through airports fast and easy.  If you are a regular International traveler, I can not recommend this more.