|This page in a nutshell:
A check to the minor edit box signifies that only superficial differences exist between the current and previous versions. Examples include typographical corrections, corrections of minor formatting errors, and reversion of obvious vandalism. 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).
By contrast, a major edit 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 not to 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.)
Users who are not logged in to Wikipedia are not permitted to mark changes as minor because of the potential for vandalism.[a] The ability to mark changes as minor is among the reasons to register.
A good rule of thumb is that edits consisting solely of spelling corrections, formatting changes, or rearrangement of text without modification of the content should be flagged as minor edits.
Below the edit summary field, there is a checkbox that says "This is a minor edit". Most browsers support access keys (keyboard shortcuts); the access key for the minor edit checkbox is i. You can mark and save an edit very quickly by using the access keys for minor edit (i) and save (s).
When filling in the edit summary, you may also press Tab ↹+Space to check the minor edit box quickly.
Administrators and rollbackers can semi-automatically revert the edits of the last editor of a page; all such rollback reversions are marked as minor by the wiki software. The intended use of the rollback feature is for cases of vandalism, where the act of reverting any vandalism should be considered minor (and can be ignored in the recent changes list).
Additionally, bot accounts usually mark their edits as minor in addition to the "bot" flag.