A minor edit is one that does not change the semantics of an article. Edits can be flagged as "minor" by checking the "Minor edit" box before submitting the edit. The purpose of flagging an edit as minor is to allow others to filter those edits out of their lists of recent changes and watch lists, perhaps because the user involved is just interested in keeping abreast of the latest information, not caring so much about grammatical corrections being made.
Whether or not a given edit is minor is not related to the number of characters added, deleted, or changed. An example of a minor edit would be changing "Tyr had two handd" to "Tyr had two hands", as that is merely a spelling correction. An example of a non-minor edit would be changing "Tyr had two hands" to "Tyr has two hands", as that changes the meaning of the sentence. In both examples, the change is of a 'd' to an 's', but one change is minor, while the other is not.
Comments added to a talk page are never minor edits, but adding a missing signature could be minor.
While there is nothing wrong with not flagging a minor edit as minor, it is poor form to flag a non-minor edit as minor. (If in doubt, the edit should not be flagged as minor.) If a non-minor edit is accidentally flagged as minor, the one who made the edit should immediately make a "dummy edit" (e.g. add an extra space between two words), not flag that as minor, and provide an edit summary along the lines of "Sorry, my last edit was accidentally flagged as minor".
Logged in users are the only ones who can flag their edits as minor. In addition, there is an option for logged in users to flag all of their edits as minor. Enabling this option is a bad idea for most editors, as it is easy to forget to uncheck the "Minor edit" box before submitting a non-minor change. The ones who could make good use of this option are the ones who only make grammatical corrections to articles, never adding any new information nor correcting the facts in an article.