A SEP IRA is a type of retirement plan for those who are self-employed or who are small business owners. At Betterment, we currently only support SEP IRAs for those who are self-employed or small business owners with no employees.
For self-employed individuals, the primary difference between a SEP IRA and a traditional or Roth IRA is the annual contribution limit. For 2018, the contribution limit to SEP IRAs for those who are self-employed is the lesser of 20% of net income from self-employment, OR $55,000 ($54,000 for 2017). The contribution limit for traditional and Roth IRAs is $5,500 for 2018 ($6,500 if age 50+).
Note that it is possible for a self-employed individual or small business owner to make both SEP IRA and traditional/Roth IRA contributions in the same year. To learn more about the advantages of a Betterment SEP IRA, you can read this article.