Conditionally format row based on cell

For example:

Let's say you have dates in D2:D100, and you want to highlight the entire row if the date in column D is older than the current date.

Select rows 2 to 100. A2 (or another cell in row 2) should be the active cell in the selected range.
On the Home tab of the ribbon, click Conditional Formatting > New Rule...
Select 'Use a formula to determine which cells to format'.
Enter the formula

=$D2<TODAY()

The $ before the column letter D is essential!
Click Format...
Activate the Fill tab.
Select a fill color.
Click OK, then click OK again.