Brazilian Juijitsi (BJJ) was popularised in the early UFC days by the Gracie family and their sporting success. Popularity has continued until now. It is a great sport. but lacks realism if you want to train in it for self defence. Instructors often claim that 9 out of 10 fights go to the ground, so BJJ is best to learn for 90% of cases. Firstly the statistic is not true. Secondly, being grounded or tied to your opponent is a bad idea. On the ground you are vulnerable to attack from additional opponents and so close to your aggressor that you wont see if they draw a weapon. However, BJJ is a great competitive sport.
Relationship to Krav Maga
Some basics of BJJ are used in the ground fighting strategies of Krav Maga. But BJJ sole range is limited to being on the ground. They are great at groundwork, they are the best in the world, but the style lacks the realism needed in modern combat and self defence training.