Acorns is an investing app that centers around the idea of investing your spare change. The app connects to your bank account and monitors your daily purchases. For each purchase you make, they round the total up to the next highest dollar amount and automatically invest the spare change for you. For instance, if you purchased a cup of coffee for $2.50, they would invest $0.50 on your behalf. 
The money is invested in what Acorns calls "smart portfolios". There are five different portfolios available that range from conservative to aggressive. The portfolios were designed with the assistance of Harry Markowitz, a man best known for developing the Modern Portfolio Theory (MPT). Each portfolio is made up of a combination of large company stocks, small company stocks, emerging market stocks, government bonds, corporate bonds and real estate stocks. The percentage of each type of investment determines whether the portfolio is conservative, moderate or aggressive. 
The app is available for both iPhone and Android. There is a web-based version as well that can be accessed through any browser. As of April, 2015, the app had 650,000 users, 75% of whom were under the age of 35.  This demographic has been particularly hard for other investment companies to reach. The success of the Acorns app is most likely tied to the fact that it is a smartphone app and that it allows people to invest relatively small amounts of money. This can be appealing for young people who may not have a lot of extra money to invest. Likewise, the fact that it takes care of everything automatically is another major selling point. There is no need to spend time pouring over your investments. Instead, you just go about your life as usual and all of the investing is taken care of for you behind the scenes.
Acorns charges a fee of just $1.00 per month for accounts that have less than $5,000 invested. For accounts over $5,000, that fee rises to 0.25% per year. Students or anyone under the age of 24 can invest for free with no fees.