Anyone who works for themselves or runs a small company has no employer plan waiting for them. They have to build one. Three IRS-recognized options come up most often: the SEP IRA, the SIMPLE IRA and the solo 401(k). They share a purpose, which is tax-advantaged retirement saving through the business. They differ in three ways that matter: who puts money in, how much can go in, and what obligations the business takes on, especially once it has employees.
This article explains those differences in general terms. It deliberately leaves out annual dollar limits, income thresholds and percentage caps. The IRS adjusts many of them from year to year, and the current figures should come from the IRS directly (see the verification list in the editor notes). The point here is to show how the plans are built so the numbers make sense when you look them up.