you will find that the cheaper pads not from toyota/lexus have the smooth/complete pad face. the groove in the middle is to allow the gases that are caused from braking to escape, along with rain/moisture. you will also find the the genuine pads have a more chamfered edge on both sides, this helps prevent brake squeal and guide away debris/water/moisture from the pad. the wear indicators on the pads are the small metal clip on the bottom of the inside pads, when the pad wears down it reaches this clip and this rubs on the face of the disc, causing a nice squealing metalic noise when you brake.
hope this helps your understanding.