The main difference is scope and authority. A Texas Food Handlers Card is a basic, roughly 30-minute certificate (no exam) required for all staff who handle open food, valid for 2 years. A Texas Food Manager Certificate is a more comprehensive credential for the owner, kitchen manager, or designated Person in Charge (PIC): it requires passing a proctored exam, is valid for 5 years, and every licensed Texas food establishment must have at least one.
Think of the Food Handlers card as the employee entry-point and the Food Manager certificate as the management credential. The manager course is longer and more in-depth (often a full day) and ends in a proctored test; the handler course is a short online lesson with no exam.
One helpful note: a valid Food Manager certificate also satisfies the food handler requirement, so a certified manager does not need a separate handler card. If you are just starting out, though, the handler card is your fast, required first step.
Just starting out in the kitchen? Get your baseline requirement done fast.