Conditional Layouts - Word of the day #53
Conditional layouts are created with layout rules, by which you can establish an interface in which certain fields are shown depending on the value entered in a previous field. As a result you are able to make your layouts dynamic - as the entire structure of a layout changes based on the field values you enter.
Note that you are not eliminating fields from the layout, you are only controlling how a layout appears based on certain field values.
Check this quick video on Conditional Layouts.