One of the new features in Excel 2010 are tools on the Ribbon that appear on demand, as you work. These tools will automatically appear as you work in your worksheet. They will be relevant to what you are doing at the time.
For example, if you are working with a PivotTable in your Excel worksheet, when you are working in your table, you will notice Options and Design tabs, along with the PivotTable Tools, will appear on the Ribbon.
Another example would be if you are adding Sparklines to your data. When you click in the Sparklines, you will see that the Ribbon now shows the Design tab, along with Sparkline Tools.
When you click away from the PivotTable or Sparklines, the on-demand tools will disappear, and the standard tabs will remain.