Ah, like a month ago was as clueless as you! I'm giving the SAT this year October iA so I've been looking it up.
Basically SAT I is the standard test made up of 3 sections which you are required to take if you want to apply to States for uni.
SAT II consists of the subject tests. You should take an SAT II if you are going to be applying to some highly competitive colleges that require two or three SAT II (subject) scores.