A tour guide (or tourist guide) provides assistance, information and cultural, historical and contemporary heritage interpretation to people on organized tours, individual clients, educational establishments, at religious and historical sites, museums, and at venues of other significant interest.  They (normally) have a recognized national or regional tourist guide qualification.
The CEN (European Committee for Standardization) definition for “tourist guide” (part of the work by CEN on definitions for terminology within the tourism industry) is: Tourist guide = person who guides visitors in the language of their choice and interprets the cultural and natural heritage of an area, which person normally possesses an area-specific qualification usually issued and/or recognized by the appropriate authority, March 2010 CEN also produced a definition for “tour manager”:
Tour manager = person who manages and supervises the itinerary on behalf of the tour operator, ensuring the programme is carried out as described in the tour operator’s literature and sold to the traveller/consumer and who gives local practical information In Europe, tourist guides (tour guide being initially a term primarily used in the US market) are represented by FEG, the European Federation of Tourist Guide Associations and outside Europe by WFTGA.
The tourist guiding qualification is specific to each and every country; in some cases the qualification is national, in some cases it is broken up into regions. In all cases it is embedded in the educational and training ethic of that country. The Art of Guiding is a skill; it is the skill of selecting information and varying it for different audiences; it is the skill of presenting it in a simple and precise way; it is the skill of allowing the visitor to see and to understand; it is a skill which, if well performed, is invisible.