First off, you can connect multiple debit and credit card accounts, which will be monitored for "round-ups". Then, instead of just taking that extra change and having it accrue over time in a low-yield savings account, Acorns will take that change and invest it for you in low Exchange Traded Funds (ETFs). Acorns will do its best to diversify investments, and hit the goal of buying low and selling high, in order to make that change into something far more substantial. Sure, there is more risk, but as always, that means there is the potential for more reward.
The one downside of Acorns is that because it is investing your money, the requirements for signing up are much higher. This isn't just a matter of moving your money from one spot to another so you don't spend it. This has the potential to make you money, which means the IRS has to get involved. So, you will have to provide full legal information including your date of birth, social security number, physical address and more.
Once you have all of that squared away, Acorns will go to work automatically investing your money for you, based on expert recommendations. The service isn't free of course. Each month, you'll lose $1 from your "round-ups" (unless you have a zero balance) which essentially acts as a subscription fee that you don't really notice. Also there is a fee of .25% to .5% per year on the total assets in your account. But, those are the only fees, you can add more money if you want, choose how aggressively to invest, and withdraw any amount at any time all at no cost.
The app is (obviously) free to download. It has been available to iOS users since earlier this year, but is now also out for Android 4.1+.