When editing a page, a logged-in user has the option to flag an edit as "minor." Use of this flag is largely a matter of personal taste. A general rule of thumb is that an edit that corrects spelling or formatting, performs minor rearrangements of text, or tweaks only a few words, should generally be flagged as a "minor edit".
A major edit, in contrast, generally performs a change that close watchers of the page are likely to want to review. Of course, if an edit performs a major semantic revision, but is limited to only a few words (for instance changing "psychiatrist" to "psychologist" or vice versa, then the edit should not be flagged as minor.