A custodian (also called a brokerage) is the financial institution that holds your investment account, executes your trades, and reports your activity to the IRS. It's the equivalent of a bank for your investments — your money and holdings sit there, and you access and manage them through the custodian's website or app.
Every account type covered elsewhere on this site — a taxable brokerage account, a Roth IRA, a rollover IRA — has to live at some custodian. Below are three well-established, low-cost providers, presented factually and in no particular order. This is not a recommendation of one over the others.
Fidelity
Founded in 1946. A full-service brokerage offering taxable accounts, IRAs, employer retirement plans, and its own lineup of mutual funds and index funds, including some with no minimum investment.
Charles Schwab
Founded in 1971. Widely credited with pioneering discount (low-commission) brokerage in the US. Offers taxable accounts, IRAs, employer plan services, and its own index fund lineup.
Vanguard
Founded in 1975. Structured so the funds themselves own the company, which Vanguard cites as the reason behind its historic focus on low-cost index investing. Offers the same core account types as the other two.