When traveling on a plane a backpack is considered a carry on as long as the backpack does not exceed 18" x 14" x 8"in dimension, inclusive of all handles, wheels, and straps.
If the backpack is larger than those dimensions then it would be considered a personal item and would need to go into the check luggage.
But if it doesn't exceed those dimensions then you can take the backpack on the plane in your carry on luggage.