AND gets more specific results because all the words must be on a web page. [Some search engines automatically insert AND between words, but it doesn't hurt for you to put it there.]

Texas AND environment will get only results that mention both the words.

OR allows you to search for synonyms or word variations in one search. Often there are other words that mean the same as your keyword. OR searches for these variations at the same time.

environment OR ecology gets results containing either the word environment or the word ecology.

NOT eliminates unwanted words. This prevents useless sites from showing in your searches.

If your topic is the planet Saturn,  you get better results by searching for Saturn NOT automobile.