Normally you can't put money in an Roth IRA unless you earn it. As of 2013, the maximum annual contribution is $5,500. If your "earned compensation" -- salary, wages, self-employment income and so on -- is less than that, you can't contribute more than your compensation. If you have a traditional IRA, however, you can convert it to a Roth even without earned compensation.
You convert a traditional IRA to a Roth by transferring some or all of the assets from one account to the other. You can withdraw money from the account and transfer it yourself, or you can direct the account trustee to do it. There's no limit on the size of your conversion. The $5,500 contribution limit doesn't apply to conversions, and neither does the rule that limits Roth contributions if you have a high adjusted gross income.
You pay tax on the conversion just as if you were withdrawing from your traditional IRA in retirement. That can stick you with a big tax bill this year, but when you withdraw from the Roth later, there's no tax at all. If you've made nondeductible contributions to your traditional account, there's no tax on converting them. Everything else is taxable. The IRS requires you use a percentage formula: if 70 percent of your account consists of nondeductible contributions, 30 percent of your conversion is taxable.
Unlike a traditional IRA, you can contribute to a Roth after you turn 70 1/2, the age at which you start taking required minimum distributions from traditional accounts. If you have to take an RMD this year, you can't convert that to a Roth -- you have to withdraw extra money to make the conversion and pay tax on both the RMD and the conversion. You never have to take RMDs from your Roth.
Conversion isn't a one-way street. If you transfer, say, $10,000 from your traditional account to your Roth, then decide it was a mistake, you can recharacterize some or all of the money back into your IRA or another IRA. You have until taxes are due for the conversion year to recharacterize. If the conversion has earned any income in the Roth, you must have the account trustee transfer that money too.