Vertical search (also called niche or specialized search) is a search system that is limited to a specific subject area, category, or type of content, as opposed to horizontal search (such as Google, which searches the entire web). A vertical system collects, indexes, and ranks information that is highly relevant only to that specific “vertical.”
Conceptually, vertical search aims to provide deeper, more precise, and more focused results than horizontal search engines, as it works with a much smaller and better-defined data set. Classic examples are image search (Google Images), video (YouTube), product search (Amazon, Google Shopping), news, or real estate. These systems have specialized algorithms and ranking factors that are tailored to the nature of the information (e.g., for video, the number of views is important, not just text content).
In the SEO context, vertical search optimization requires specific data structuring (via Schema Markup) so that content can be properly identified and presented in these specialized sections of the SERP, becoming a Rich Result.