For most journeys you book directly: you see the fixed price and your journey is booked straight away.
For longer journeys, the booking form also shows a Request a quote button for each vehicle. A quotation is useful for several stops, groups, more than one vehicle or special requests. Requesting a quotation is free and is not yet a booking: your journey is booked once you have accepted the quotation online.
How both work, step by step: How to book a taxi.