Adobe Illustrator has a wonderful assortment of tools for adding fill colors and patterns to paths and objects. However, those fill options aren't as plentiful when it comes to adding fills to type. Workarounds for these limitations are available. One method allows you to use type as a mask to apply a gradient to your text. Here's how to do it:
Create a word or phrase of text.
Use the rectangle tool to create a rectangle about the size of your text. Apply a gradient fill to this rectangle.
Drag your type on top of the filled rectangle, making sure that the text is on top. (Object > Arrange > Bring to Front)
Select both pieces, the type and the rectangle.
Choose Object > Clipping Mask > Make.
Click anywhere outside your type to see the effect.
You could use this technique to create unique headlines for a newsletter, or in your company's logo.