A 'trade' is a transaction in which two teams designate specific players and swap them. Trades can happen before, during, or after the draft, depending on the type of fantasy league. In keeper and dynasty leagues, draft picks can be used as trade compensation.
The overall goal of a trade is to get a player who can help your team without giving up someone who benefits your roster as well. If both parties are competent fantasy football players, it is hard to become anything other than marginally better due to a trade. A beneficial trade for example could be giving up a bench player on your team for a bench player from another, however, if they play different positions you could be able to start the player that another team had no use for.
'Fleecing' is the act of taking advantage of a poor team owner by conducting a lopsided trade. Many leagues have a veto process in which the owners can disallow a trade if they believe it to be too one-sided. Some leagues even have a commissioner veto as a form of executive power to ensure one team does not cheat their way to success by undermining their opponents.
An example of fleecing is by trading 'damaged goods', or a player who is injured, unbeknownst to the trading partner. However, injured players can be traded fairly if they are supposed to come back later in the season and an owner is attempting to make a long term investment.
The 'trade deadline' is the last day of the season in which a trade can be processed. Usually this occurs around week 12 or 13 of the NFL season, and ensures that teams who are already eliminated from playoff contention cannot trade all their good players to playoff bound teams. Leagues can choose whether or not to have a trade deadline.