To give you an easy to understand definition of business verticals or vertical market; It is in a way the equivalent of niche but customer wise.
Meaning, a niche business is a business that focuses on a small subset of a bigger industry. Verticals are businesses that focus on a niche customer base, and try to offer them complementing products and services.
Now let’s jump to the standard definition: Vertical markets, are business niches where vendors serve a specific audience and their set of needs.
Example of verticals: Gym goers, they need gym clothes, protein shakes, sport shoes, protein bars.
A business or set of businesses focusing on vertical markets, reduces the customer acquisition cost to almost zero. Thus less need to advertise since the customer is already acquired in service A.