An IRA, in its various forms (Traditional, Roth) is just a vehicle with special rules that you 'house' your investments in. These investments can be in the form of index funds (VTSAX), individual stocks, mutual funds, ETFS, Bonds, cash, and so forth. Think of it as choosing what kind of basket you're choosing to put your eggs in (eggs being VTSAX).
The rules for IRAs consist of many flavors, including annual contribution limits, early withdrawal penalties, and tax advantages/disadvantages. I don't happen to have any useful links handy, but I suggest doing some Google searches to get a better understanding of the nuances between a Traditional IRA and a ROTH IRA.
The common strategy I hear is to max out your tax advantaged investment accounts such as your 401k ($18,000 if you have one) and IRA ($5,500). The benefit of maxing out a 401k is that you will not be taxed on that money at the end of the year since it is deducted prior to payroll tax from your check. A portion of your post-tax IRA contributions can also be tax-deductible. After you've filled up these accounts (or baskets, as I've nicknamed them) each year, you can pour money into a taxable brokerage account.
Also, it is smart to at least contribute up to the amount that your employer will match in your 401k. That's free money.
Oh, and don't forget about HSA's. Those are funded with pre-tax dollars from your paycheck as well if your employer offers it. Health Savings Accounts are exactly what they sound like -- pre-tax money you set aside to cover health expenses. It's pretty broad and covers things like medical, vision, and dental expenses. I hear it can even cover the cost of Band-Aids, cold and flu meds, and contacts. Why put money into it? Essentially a HSA gives you greater purchasing power. You're using money that wasn't taxed rather than money out of your paycheck, which was taxed. Plus, the brilliant folks around here know how to access that money in other legitimate, creative ways too.
Separately, you can open a brokerage account and purchase VTSAX through it. I have a taxable brokerage account with Vanguard and do not pay any commission fees when I purchase shares of VTSAX. My previous financial institution was charging me $35 each time I bought VTSAX, so make sure to do your research on fees & commissions. Because (this portion of) my holdings are not in a tax sheltered account like an IRA or 401k, I pay taxes on the dividends and capital gains each year.
I suggest poking around Vanguard.com
