I am creating a business directory website when everyone comme and add his business to the website to allow people to find him later. A business has many fields (categories, products, services, name, adresse, pictures, comments...) I am wondering which field i must use for the search ? I tried to understund how yelp has deal with this problem but i couldn't understund which field is used in Yelp website !
I am assuming you are talking about the main "headline" search rather than a detailed or advanced search and, if so, I would not restrict myself to one field - look at the type of thing done by sites like Yell.com for example
They allow you to enter a business type or company name in one field and then a location in the second field - so for example, show me a carpenter in London or a company called ABC Construction in Manchester
Both scenarios are dealt with by the same form meaning the customer does not need to decide or click any additional buttons - they just enter what they want and the logic looks after it in the background.