A check to the minor edit box signifies that only superficial differences exist between the current and previous versions. Examples include typographical corrections, formatting and presentational changes, and rearrangements of text without modification of its content. A minor edit is one that the editor believes requires no review and could never be the subject of a dispute. An edit of this kind is marked in its page's revision history with a lower case, bolded "m" character (m).
When filling in the edit summary, press Tab ↹+Space to check the minor edit box quickly.
By contrast, a major edit is one that should be reviewed for its acceptability to all concerned editors. Any change that affects the meaning of an article is not minor, even if it concerns a single word; for example, the addition or removal of "not" is not a minor edit.
Because editors may choose to ignore minor edits when reviewing recent changes, the distinction between major and minor edits is significant. Logged-in users can set their preferences to not display minor edits. If there is any chance that another editor might dispute a change, the edit should not be marked as minor. (If an editor considers a change to be minor yet conceivably controversial, it is always possible to include "minor" in the edit summary without ticking the "minor edit" box.)
A good rule of thumb is that edits consisting solely of spelling corrections, formatting changes, or rearrangement of text without modification of content should be flagged as minor edits.