TSA Precheck gives you access to the Precheck lines at U.S. airports.  Among other things, this means you don’t have to take off your shoes, nor remove your (still limited) liquids ziplock from your bag, nor remove you computer from your bag.  This makes things go much smoother and faster.  Global Entry includes TSA precheck  and also expedited customs screening when re-entering the U.S. (you don’t have to fill out any forms, and you can use a kiosk to get through, which is very fast).

The costs are very similar — TSA Precheck is $85 and Global Entry is $100 (for a 5-year membership).  If you don’t take international trips very often, use this Tool from the Department of Homeland Security to see which program they recommend for you.

When you apply for Global Entry at the Trusted Traveler Program website, you are screened, and then you have to meet in person with a Customs officer.  Once approved you get a membership number that you fill in in the  “known traveler number” field on many airline websites (this gives you TSA Pre-check for flights with them).  It is not automatic that you can just buy a ticket and it knows that you have TSA Pre-check so make sure you ill that out in any profile page when you are buying or you won’t get it.

I had been to Cuba when I applied for Global Entry, and I did have to provide the certification that I went on an approved trip.  I will have to re-supply that documentation at each renewal, or I can’t get Global Entry anymore.  So be sure to check all the requirements for GE and make sure to follow them.  You will also need to be fingerprinted at the interview mentioned above.