"Green energy" and economics results with both terms appearing anywhere in the document. The quotes around "green energy" searches for the phrase "green energy" instead of individual words.
Real-life use: REFINES your search. You'd do this search to get green energy articles that concentrate more on the economic side of green energy, for example.
Rejects articles that contain a word or phrase. Example: You're interested in animals but NOT birds.
Real-life use: REFINES a search. For a topic with many subcategories, takes out what you don't want.
Green energy or solar. Gets articles with either "green energy" or solar.
Real-life use: EXPANDS your search. For example, when your topic is called a couple of different things.
Tip: Use advanced searching techniques like the parentheses here: (baseball or football) and (advertising or marketing)