What’s the difference between a logo, and a submark/watermark?

A submark is a variation of a logo, used in situations when the main logo is not the best fit. It is visually similar enough to be associated with your main logo, but more simple in design with somewhat proportionate dimensions making it easier to use on things like social media, packaging, etc.

A watermark is, again, a variation of a logo and usually used on an image, with a great deal of transparency. So, it's still possible to visualize its presence without interrupting or preventing vision of the image that it protects.

The image to the right show all three. The submark is on the white business cards, the logo is on the pink business cards, and the watermark is on the bottom right hand corner of the photo.

Keep in mind, not all logos/businesses warrant submarks & watermarks.