The SAT and ACT are two very different college entrance exams. Both are nationally administered and are used to help universities evaluate applicants. The SAT is essentially a three-part aptitude test (Reading, Math, Writing) while the ACT is more of an achievement test with four core sections (English, Math, Reading, Science) and an optional Writing section. Most universities accept both exams and have no preference which exam is presented for admission.
The link to the chart below will help you understand the structure and content of the SAT and ACT.