For instance, you can type pets NOT cats and get pets of all kinds except cats. If you want to find black holes and type it in as two words with a space in between, most engines automatically do an AND search. The result will be all the pages where black and holes appear together but not necessarily as a phrase. Read the help for each search engine to find out which way it groups phrases. Some engines ask you to specify words in the order of most importance.
Some engines take the percentage of hits for each keyword and put the highest percent at the top.
Again, read the help to find out if weighted searches are possible and how to do them. It will save you a lot of time. A new feature of some search engines is the ability to search for meaning rather than just keywords. This can mean searching for the Sistine Chapel might also bring up hits for Michaelangelo and the Vatican. It also means that you might come up with synonyms for your topic that you hadn't considered.