TSA PreCheck and Global Entry are both Department of Homeland Security Trusted Traveler Programs, but they speed up different parts of a trip. TSA PreCheck provides expedited security screening for flights departing from U.S. airports—typically dedicated lanes where eligible travelers can keep shoes, belts, and light jackets on and leave laptops and liquids in their bags.
Global Entry focuses on expedited U.S. customs processing when you reenter the United States after international travel (usually via a kiosk). Global Entry membership also includes TSA PreCheck benefits, so one enrollment covers both outbound security and inbound customs for many travelers.
Per the TSA, travelers who fly internationally about four or more times a year often prefer Global Entry; mostly domestic travelers may find TSA PreCheck alone sufficient.