A tout is any person who solicits business or employment in a persistent and annoying manner (generally equivalent to a solicitor or barker in American English, or a spruiker in Australian English). According to the American Bar Association, touting occurs when a person advertises, promotes, or otherwise describes a security for sale without disclosing that the person is being paid to do so.
An example would be a person who frequents heavily touristed areas and presents himself as a tour guide (particularly towards those who do not speak the local language) but operates on behalf of local bars, restaurant, or hotels, being paid to direct tourists towards certain establishments.
In London, the term "taxi touts" refers to a kind of illegal taxicab operation which involves taxi drivers (or their operator) attracting potential passengers by illegal means--for instance, calling out travellers, or fetching them and their luggage, while parked in an area where taxi drivers must wait in their vehicle. They may charge exorbitant fees upon arrival, possibly using threats to ensure payment.
"Ticket tout" is a British term for a scalper, someone who engages in ticket resale for more than the face value of the ticket. In recent years some British ticket touts have moved into Internet ticket fraud.
A Shop Tout is someone who is engaged by a shop to loiter outside their office, sometimes outside the building, or outside their competitors' shop. The Tout then promotes the services to a passer-by and then escorts the person back to the shop, where they are paid a commission for each person that is brought back. The practice of touts working on the street to attract customers to night clubs and bars is very common in the entertainment tourist areas of Japan, particularly those of Roppongi and Kabukicho, Turkey and Spain.
Taxi Touts: These are completely illegal and are bad news, avoid them like the plague. They wait for unsuspecting tourists who do not know any better, usually in places such as London airports and railway stations. They will appear to be friendly and helpful, carrying your bag from the train station outside to the car. Remember that mini cabs cannot pick you up on the street, so if it looks like a private car, not a big black taxi, then you should refuse to get in. Take your bags and leave. There have been cases whereby tourists have been taken only a few blocks and then charged £60 or more. If you refuse to pay you could really be in trouble as they tend to have some very big friends that could land you in hospital.