A mark can be anything used with a product or service that serves to identify the source of the product, service, group, or certification. For example:

• Logos, e.g., the McDonalds golden arches

• Alphanumeric combination, e.g., ABC, V-8

• Slogans, e.g., INTEL INSIDE

• Colors, e.g., Pink (for insulation)

• Musical notes, e.g., NBC's chimes

• Product configuration, e.g., the hour-glass shaped Coca-Cola bottle.

For more information, click here for the USPTO's explanation.