Dissent is a verbal and/or physical show of disagreement with the referee or assistant referee's opinion/decision, and with no offensive/insulting/abusive language included.
Verbal dissent comes in many phrases, the most regular being the likes of "You're having a laugh, ref" and "That's a joke, referee - how can you give that?" (usually loudly and with feeling!)
Gestures which often occur in this capacity include slamming the ball down when a free kick or throw is awarded to the opposition, an obvious gesture (e.g. arms raised to the heavens) showing disagreement with a decision, etc.
The sending off offence is the use of offensive, insulting and/or abusive language.
Your level of acceptance will be different to that of other referees, and you may elect to warn/advise players or club officials early that dissent and/or language is unacceptable (e.g. "If I hear that sort of language again you will be going off")
Always consider whether the language is acceptable in the context of the game, and the surroundings - e.g. if there is a children's play area adjacent, a lot of spectators who may be unhappy with the language being used, etc., you can use that as a reason for players to reduce/stop the language usage.
The language does not need to be aimed at you - may be at a team-mate, the player himself of herself, an opponent, or the world in general - to be a dismissal offence.
The same approach applies for coaches, managers, etc.
Set your standard publicly early in the game - "cut the language, 5" - as this then sets up the players for any action later.
The question of pre-match issues of this type is explained fully at
http://www.thefa.com/football-rules-governance/lawsandrules/laws/football-11-11/law-5---the-referee, and this shows what to do with any problems on arrival, and then the period after entering the field of play before the start of play.
Any post-match comments/gestures/unacceptable behaviour can be reported as misconduct after the match through whichever system is in place on your league.
I hope that helps, JRef