Vertical Marketing simply means promoting your products and services to a specific industry niche which is related but not identical in scope to the initial core audience you intended to serve . Most solutions that are offered by marketers cater only to one general group.
For example, you may be selling a software that keeps track of how people spend their time during the day. A few changes to the core software (or even just to the marketing/advertising copy) could enable a marketer to promote their product to housewives specifically who may be interested in making the most of their time during the day.
To achieve success in vertical markets, it is essential to become active in their online communities and explain how the solution provided will solve a specific problem or save time, in a specific industry.