SMD stands for "Surface Mounted Diode", which is the latest LED bulb technology. The LEDs are "chips" mounted to a tiny curcuit board. This produces a brighter blub than before, with more uniform brightness between colors - for less money.

The C7 SMD bulbs have 2 LED chips per bulb and the C9 SMD bulbs have 3 LED chips. These will screw into your existing C7/C9 strings.

LEDs are directional by nature. The faceted bulbs spread the light out sideways more than the smooth.