More often than not, bulbs do not burn out but rather short out or get broken.
Advanced light strands bypass bulbs which short out, break or are removed. There are two types of advanced strand technology. Big box retailers tend to sell strands which use a "fuse bulb" where the first bulb in the strand acts as both a fuse and a bulb. Unfortunately, if this bulb is damaged the entire strand is ruined as the fuse bulb cannot be removed or replaced.
Slightly more expensive advanced strands have a chip in the bottom of EACH socket which keep the strand lit. These chips typically add about 6 cents per bulb to the price of the strand. Examples of this would be Stay-lit, Dura-Lit and PerfectLit strands.
Source: Bill Quinn, Christmas Tree for Me