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 outsource.
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%.