The classic free online text on LaTeX. The second one is used to write expressions that are not part of a text or paragraph and are therefore put on separate lines. LaTeX is obviously pretty good at typesetting mathsit was one of the chief aims of the core TeX system that LaTeX extends.
In 1996 they were updated by Thomas McKibben to work with LaTeX2e which had been released in 1994.
The switch command centering will also produce centred text but the behaviour is different. It is sometimes useful to add text in Latex formulas or equations. Also that each equation is separated from the one before by an. For example open a new source file in TeXnicCenter and type or copypaste the following.