The government requires that by April 1 following the year in which you turn 70.5, you are required to being taking minimum withdrawals from your IRAs, excluding the Roth IRA. The amount you are required to withdraw is based on a calculation (use this calculator) and that value is an annual number.
The example calculation is based on a $100,000 account balance and an 8% rate of return (based on a 27.5 year remaining life expectancy) and it requires that you take a minimum $3,649.64 withdrawal each year. The calculator is based on the published rules as of April 2002 and the calculator was last updated January 2006.
If you don’t take the required minimum withdrawals, then you are penalized 50% on the amount you were short by. So let’s say you were required to withdraw $5,000 and you only withdraw $1,000, then you actually owe a penalty of $2,000! So, be sure to take the minimum withdrawal or you’ll face a penalty but you can do whatever you want with it, including reinvesting it into the market (even into your Roth IRA if you meet the restrictions).