Justification works by adding white space between the words in each line so all the lines are the same length. This alters the ideal spacing of the font, but in paragraphs of reasonable width it’s usually not distracting.
If you’re using justified text, you must also turn on hyphenation so you don’t get gruesomely large spaces between words, as shown in the example below.
Justification is a matter of personal preference. It is not a signifier of professional typography. True, most (but not all) books justify the text. But most major US newspapers use a mix of justified and left-aligned text. Same with magazines.
Keep in mind that the justification engine of a word processor or web browser is rudimentary compared to that of a professional page-layout program. So if I’m making a word-processor document or web page, I’ll always left-align the text, because justification can look clunky and coarse. Whereas if I’m using a professional layout program, I might justify.
But the choice is yours.
If you’re using justification in a high-end page-layout program, you’ll have the choice of distributing the extra space in each line between words or between letters (or some combination). Please—put it between the words. Type designers spend a lot of time getting the spacing between characters right. (Some of us would even say spacing is the essence of type design.) If your character spacing is changing from line to line, your justified text block will look horrible. For another instance of this problem, see metrics vs. optical spacing.