what is gd&t
GD&T has been around in one form or fashion since the mid 40's. it's purpose is to eliminate misinterpretations of design data and to allow the engineer/designer to incorporate design intent into the engineering documents. it first came from the military aerospace world, but soon the automotive world and later other industries saw the advantages. see these links for more information:
now for a simple example:
suppose you have a square hole pattern in a part. you have dimensions from the left side to one hole centerline and a dimension from center to center of the holes. you have a dimension from the bottom edge to the center of the lower hole centerline and a dimension between centers vertically. all dimensions are +/- .005. this gives you a square target area that is .010" on a side. which means your worst case error is at 45 degrees and the magnitude is .014, not .010! this makes tolerance stackup calculations pretty difficult.
now take the example of GD&T using a simple positional control of .010, dimensions are in the same places as before but basic. and the two sides are defined as datums A and B. this gives a circular target zone of .010 diameter no matter what angle you look at it from.
things get even more difficult when you figure the tolerance zone for a bolt circle hole where you have a angle and a diameter and that diameter has to be concenteric with another circular feature. now try to accomodate perpendicularity of the fasteners going through the holes. Things can get pretty crazy real quick. GD&T has tools to address all these issues, conventional drafting does not.