You may be asked to fill out a W9 form by your bank, investment advisor, or pension fund when opening an account. These various institutions require their clientele to complete this form to identify American citizens to the US government.
If you are not a U.S. citizen or filing for your non-U.S. company or partnership, you should fill out a W8 form instead.
The W9 form is straightforward and only requires you to provide your name, address, social security number, and entity type (individual, partnership, corporation, etc.)
An example of this form can be found below: