What is thrush and how do you know if your baby has it?

my daughter is turning 4 months next thursday and i noticed to day after recieving a weekly newsletter from babycenter to look out for thrush. i stopped feeding at 2 months and 3 weeks and she has been on formula since, and i noticed today as she was smiling and her mouth was wide open and i noticed the roof of her mouth was white? now i know that lugo? i think it is is when the gums r grey but, im not sure what it is, she has an appointment next monday the 25th. but cld someone help me out now?