The SAT is named because it was originally a "Scholastic Aptitude Test" more of a general intelligence exam with its roots stretching back to the military and World War I. The ACT on the other hand, was created more recently (1950's) and was initially designed as an end-of-high school competency exam that tested how well a student had learned basic high school level content--math, reading, logic, grammar and vocab (essay writing option)
Over the years the ACT has become the preferred exam with more students now taking it over the SAT--this trend was made possible because all colleges and universities now accept both exams. However, over the years, and in response to these shifts, the SAT has moved to make their test more and more like the ACT--moving away from a general intelligence exam and towards a high school competency exam. With the changes implemented in the March 2016 administration, the SAT made what is probably its last move to become an actual high school competency exam.
Nevertheless, the student should bear in mind that the NEW SAT has roughly 90% of the content as the OLD exam--in other words, the vast majority of the content remained the same as before. That's because the skills they test don't change. Most of the changes are form and not substance. The SAT and the ACT test the same five (5) basic skills: math, reading, logical reasoning, grammar and vocab.