There are 5 belts in Brazilian Jiu-Jitsu for adults: White, Blue, Purple, Brown and Black. Children under 16 receive Gray, Yellow, Orange and Green belts which are intermediate belts between White and Blue.
White stripes on belt is a way to measure progress between belts. Once a student receives 4 stripes on his/her belt, he/she is closed to be promoted to the next color belt.
Stripe and belt promotion are awarded for training time, Jiu-Jitsu knowledge and skills developed, and most important how students carry themselves on and outside of the mat (ie: behavior, attitude).
More information about the IBJJF graduation system here