Adobe Premiere Pro CS4

PDF (adobe.com)

Compositing

Creating transparency and solid colors with keys and mattes

Apply a key to a clip
Make a color transparent with Color Key
About mattes
Define transparent areas with Image Matte Key
Replace a static background with Difference Matte
Move or change the transparent area with Track Matte Key
Mask out objects with garbage mattes
Create a solid color matte
Remove a black or white matte

community help (adobe.com)