Google, the company that helped make it fun to just sit around surfing the web, is jumping into the fitness-tracker business with both feet, buying Fitbit for about $2.1 billion.

The deal could put Google in direct competition with Apple and Samsung in the highly competitive market for smartwatches and other wearable electronics. But it also raises questions about privacy and Google's dominance in the tech industry.

The company's announcement Friday came with a promise that it won't sell ads using the intimate health data that Fitbit devices collect.

Fitbit is a pioneer in wearable fitness tec...