The SAT, the PSAT/NMSQT, PSAT 10, SAT II Subject tests, and AP standardized exams are created and regulated by a private company called the College Board (collegeboard.org). The ACT is run by a different company, ACT (act.org). 

SAT: The SAT is an exam used by colleges to predict how well a student might do in the first semester and to compare that applicant with others. The SAT tests reading, writing, and mathematical aptitude.

PSAT/NMSQT: The PSAT/NMSQT is a preparatory, or practice, exam for the SAT. This is also the qualifying exam for the National Merit Scholarship.

PSAT 10: The PSAT 10 is practice for the SAT for 10th-grade students.

Advanced Placement (AP): The Advanced Placement (AP) program is designed to offer college-level coursework at the high school level. The curriculum for AP classes is approved by the College Board and is designed to prepare students for an AP exam offered in May. Colleges may grant college credit for a high score on these exams.

ACT: The ACT is an exam very similar to the SAT and is used for the same reasons. The major distinction is that the ACT includes a Science section.

Pre-ACT: The Pre-ACT is a practice exam for the ACT.