The "verticals" of Information Technology are considered to be the IT Professional Disciplines that make up the Information Technology industry. IT professionals operate within or across these vertical IT Disciplines. Unlike the horizontals of IT, which include the different types of technologies, the verticals focus on how IT organizations and professionals operate and the things they have to worry about, on a day-to-day basis.
Based on researching the web, the most complete inventory of vertical IT Disciplines seems to be that which is published by the International Foundation for Information Technology (IF4IT). The inventory provides definitions, descriptions and content on these vertical areas of IT.