Keyword searching is what most people are familiar with. It is what most people do when they search for something in Google or Duck Duck Go or even a library catalog. Keyword searching is basically entering a few important terms into the search box and allowing the search engine to find all items that contain all of those terms.
Author or title searching is fairly straight forward, you enter the name of the author and some bits of the title in the search box and allow the search engine to do its magic. Unless you are using the advanced search feature of a search engine, this really is just a keyword search and the search engine is just looking for a match to all parts of your query that is the name and title of the book.
Phrase searching is one step better than just keyword searching. This is when you have an exact phrase that you are looking for. It may just be two words, but they need to appear in the right order. To perform a phrase search, you just put the entire query into quotation marks. This will force the search engine to find that exact match.