If you want to retire comfortably, open an IRA and start growing your money. Source: 401kcalculator.org

According to the Social Security Administration, the average retirement benefit paid in 2013 was $1,451 per month to men, and $1,134 per month to women. That's good for about $31,000 per year for a retired married couple, which is a far cry from the roughly $49,000 U.S. median annual income. We're talking about a $18,000 income gap between Social Security and the median household.

Whether you're a few years or a few decades from retirement, you can bridge the gap between Social Security and what you'll need to live comfortably in retirement with an IRA, or individual retirement account. This is a personal account that you can fund each year, allowing you to save on taxes and building income for when you retire. Before you open that retirement account, it's vitally important that you understand your options and the implications. 

Here's a closer look at everything you need to know before opening an IRA

What's an IRA? 
In short, it's a savings and investment account that allows you to set aside money each year -- up to $5,500 for 2015 -- with some big tax benefits. There are two kinds of IRAs, with slightly different benefit structures.

A traditional IRA is most common, and offers a number of benefits, including potentially deducting contributions from your taxable income the year you make them. In other words, if your total income is $50,500 in 2015, and you contribute $5,500 to a traditional IRA, your taxable income would be $45,000.

If your employer offers a retirement program like a 401(k), you might not be able to deduct all of your IRA contributions from your income. Refer to the table below to see if you qualify for a deduction in 2015 based on your adjusted gross income, or AGI. Nonetheless, regardless of whether you qualify for a deduction, those contributions would grow tax-free until you begin taking distributions in retirement, at which point the distributions would be counted as regular income, and taxed at your nominal tax rate. 

Source: IRS website.

The Roth IRA is in some ways superior. You can contribute the same $5,500 in 2015, but contributions are not deductible from your income. However, once the money is in the account, it's never, ever taxed, as long as you take distributions after retirement age. In other words, you can create a source of completely tax-free income in retirement with a Roth. 

You may also be considering a "rollover IRA" if you recently left your employer but want to take your 401(k) with you, or want to consolidate several old 401(k)s. By rolling over your funds, you don't pay taxes and keep growing your money tax-free until taking distributions in retirement. A rollover IRA is basically a traditional IRA, but brokers use this designation to make it simpler for consumers to pick the right account to rollover funds. 

Start now, even if you can't max it out 
Even if you can't contribute the maximum amount each year, it's worth starting as soon as you can, and with any amount you can afford. You'd be stunned by how big a difference it can make:

Based on $150 per month, increased 3% annually. Returns based on historical stock market averages. Source: Author

Even small amounts can add up big over time. And that's before considering the tax benefits, whether while still working or in retirement. 

Which IRA should you open?
The answer is, "it depends". It may seem like the Roth makes the most sense -- tax-free retirement income is a no-brainer, right? -- but it really boils down to a combination of your entire current situation, your current income, and tax status. 

If you're doing a rollover, you'll need to open the same kind of account as the one you're rolling over. If you have a Roth 401(k) -- which are less common but growing in popularity -- you'd need to rollover your Roth (401)k to a Roth IRA. And if you already have a traditional IRA, you can rollover your 401(k) into that account, without opening a new "rollover IRA".  The good news is your online broker (find a good one here) can help set up the right kind of account. Be sure to shop around, as many will give you bonus money, depending on the size of your rollover. 

If you're not doing a rollover, a Roth is the best choice for most folks, since the majority of workers have a retirement plan through their employer, and fall within the adjusted gross income limits set by the IRS each year. Here are the 2015 limits:

Source: IRS website.

If your income is above the amounts above, you can't contribute to a Roth, but you can still contribute to a traditional IRA as there are no income limits. You won't be able to deduct the contributions from your income if you have a retirement plan at work, but it will still grow tax-free. 

If you don't have a retirement plan at work and fall below the income guidelines in the table above, you may want to consider a traditional IRA over the Roth, especially if your effective tax rate today is likely to be higher than it will in retirement. If that's the case, the tax savings today are worth more than than tax-free income in retirement.

This only applies to a small number of mostly self-employed people. If you're in that category, you should consider opening a self-employed 401(k), since you can contribute a much larger amount to a 401(k), as much as $51,000 in 2015, based on combined employer and employee contributions.