Billion-Dollar Ideas

There are more than 25 technology start-ups valued at over $1 billion by private investors, and perhaps as many as 40. While this is a mark of success for the individual companies, it is also inspiring some worry. Do these high prices suggest a disastrous tech bubble and foreshadow a crash on the scale of the one just over a decade ago? Or is it a real sign of sweeping change, as cloud computing, smartphones and social networks affect not just tech, but also the broader economy? Here is a look at a few markets that are creating numerous billion-dollar companies.