Submarks and alternate logos are variations of your main logo and give you more flexibility in where you can use your logo.
Submarks tend to be a simplified version of the logo and are generally used when your full logo doesn’t quite fit for example, on social media or stamps.
Alternate logos are other variations of the main logo provided in a different orientations, for example, centered or left aligned. Again, these are generally used when the main logo isn’t the best fit.
Logomarks are the visual elements or icons within your logo (think the swoosh in the Nike logo) while wordmarks are the text elements within your logo (so the word “Nike” in the Nike logo).