What version of Ubuntu are you using? In 10.10 there is a glitch which affects
many webcams, including my cheap Chinese unit. The fix is in the Q&A column of
Full Circle Magazine, issue 44.

Does your webcam work in Cheese?

Some people find that they have to start Skype by using this command:
bash -c 'LD_PRELOAD=/usr/lib/libv4l/v4l1compat.so skype'

The command lsusb will identify your webcam. Mine says:
Bus 003 Device 002: ID 0ac8:303b Z-Star Microelectronics Corp. ZC0303 Webcam

Now the odd part: the "easy" way to solve webcam problems is to Google
"0ac8:303b", in the case of my webcam. That's because the people who can give
solutions, will include that information in their answer.