A consulting or consultancy firm is a business of one or more experts (consultants) that provides professional feedback to an individual or an organization for a fee. The types of firms vary, such as technology and advertising firms. 
Consultancy firms target company executives and provide them with consultants, also known as industry-specific specialists and subject-matter experts, usually trained in management or business schools. The deliverable of a consultant is usually advice or a recipe to follow to achieve a company objective, leading to a company project.
More and more consulting firms are complementing the strategic deliverables by providing the means to implement the recommendations, either with the consultants themselves or by providing technicians and/or experts. This has opened up new markets for these companies. This is called outsourcing.
Consulting services are part of the tertiary sector and account for several hundred billion dollars in annual revenues. Between 2010 and 2015, the 10 largest consulting firms alone made 170 billion dollars growth revenue and the average annual growth rate is around 4%.
The segmentation of advisory services varies widely across organizations and countries. Categorization is unclear, in part because of the upheavals that have occurred in this industry in recent years.
One approach is to separate services into four broad service delivery families, considering the managers they are targeting:
- Services related to the company's overall strategy, which are addressed to the CEO,
- Services related to marketing, communication, sales and public relations, which are addressed to the CMO,
- Services related to management, financial management, taxation, accounting, compliance with regulations, for the CFO,
- Services related to the company's operations, including information technology, intended for operational management, which may be different depending on the industrial sector (technology director, plant managers, operations directors, Research and Development managers), for instance COO and CTO.
There are different types of Consulting Firms serving different sectors. They mainly fall under the following fields:
- Architecture and Engineering
- Financial services
- Health care
- Hotel and hospitality industry
- Human resources
- Information technology
In addition to the above-mentioned fields, there are consulting firms that serve niche sectors such as
- advertising/marketing/public relations consulting
- environmental consulting
- entertainment/media consulting
- energy consulting
- consulting in politics and the public sector
- real estate consulting
- recycling consulting 
- Financial services
- Information technology consulting
- Management consulting
- Human resource consulting
- Hospitality industry