The search engine does not care about the case of your text. [ hotel restaurant spa ] will search pages talking of hotel, restaurant and spa in the same page.
The double quotes (") allow you to search an exact phrase, respecting all caracters including spaces.
If you are looking for a place to eat a bouillabaisse in Saigon, do not type "Where to eat Bouillabaisse in Ho Chi Minh?" but just "bouillabaisse ho chi minh".