Logos and watermarks work with a similar goal - to give identity to your brand. However, the minor difference between the two is that logo

Logo is a symbol or other small design adopted by an organisation to identify its products, uniform, vehicles, etc.

watermark is a a faint design made in some paper during manufacture that is visible when held against the light and typically identifies the maker.

To know how to apply logos to your account, click here.

To know how to apply watermarks to your photos, click here.