SN2 is a concerted reaction however, SN1 occurs in two consecutive steps.
SN1 and SN2 are both nucleophilic substitution reaction that differ in they way they occur. SN2 is a concerted reaction however, SN1 occurs in two consecutive steps.
The answer will be too long if I will explain all the aspects of SN1 and SN2, that is why, I recommend that you watch this video which explains in details the differences and similarities between SN1 and SN2.
Here are two videos on SN1 and SN2 practice questions: