A guide to the rapidly expanding world of digital dollarsIf the pundits are to be believed, currency will soon be a relic of the analog age, as quaint as a typewriter. In place of bills and coins (and in many instances, plastic credit cards), behold the smartphone. Analysts predict the mobile payments category will grow to $630 billion by 2015. “We’re starting to see this gaining traction, but it will be years before every merchant and every consumer accepts it,” says Todd Ablowitz, president of mobile payments consulting firm Double Diamond Group LLC. “What matters to retailers is this: Consumers love their mobile phones and they react to offers, especially younger consumers.” Mobile transactions come in a few garden varieties. Here’s what you need to know:1. Mobile acceptance Square Inc., co-founded in 2009 by Twitter co-creator Jack Dorsey, is the best-known name in mobile