Like with the offense, there are hundreds of defensive tactics that exist in Ultimate. Here are the essentials:

 

Man on Man Defence

The most simple defensive strategy is a man on man often called simply "man".

Players will be matched up with opposing players by height, speed and ability and when playing mixed ultimate usually by gender.
A certain force will usually be called on the line prior to the pull. All the downfield players can mark up on the open side, the side which the force should make the handler throw, of their man. Their primary job is to stop their player getting into space to stop them receiving the disc.

The closed side, or the break side, however can't be completely ignored.  It is usual to try and stay about 1-2m on the open side of your man so you aren't too far a way if a throw comes to the break side.  

When playing "man" against a stack offence it is common for players at the front of the stack to mark "under" their man, whilst players near the back will mark deep of their man.  It is a good idea to try and postition yourself so that you can see the thrower as well as your man.

Good defenders will often try to anticipate any cuts by their players and will stay on the balls of their feet to maintain balance and be able to react to cuts and fakes.

"Man" defense is very reliant on holding the force, i.e. no throws going to the closed or break side.  As a result often 2 defenders will look to work as a pair in "sandwich" defence.  One defender will stand on the closed side and one on the open side and they will take which ever offensive player goes which side.

Swapping of players which you are marking like this is called a "switch" they can be used deep and under as well as open and closed.  These defensive strategies are very reliant on good communication as if the other player doesn't pick up on the switch a player will be open, often for an easy scoring pass!

Sometimes the first man marking in the stack will "poach" in the passing lane on the open side to try to put pressure on the thrower and block open side cuts.  Downfield players will switch up as required if the offensive player who is being poached cuts long downfield.

If an opposing team plays through one particular very skilled handler, although the opposition may have quicker or more athletic players on their team, you might put your biggest most athletic defender on them to try to limit their impact on the game.

 

Directional Force

If it is windy the force may be set to trap the other team on the downwind line or force throwers to try and throw into the wind.

Sometimes "FM" or force middle is called to switch the direction of the force towards the centre of the field to compact the opposistion into the middle of the field back towards the rest of the defense.

Force "line" can be used to try and trap the opposition on the sideline - a good option if you're a more athletic team than the oppostion.

"Straight up" force is also sometimes used - communication is vital; often "left hand" "right hand" is heard being called by the sideline to let the force know which side the cuts are heading too and thus where the throw is likely to be made.

You could change the direction of the force after 5 passes to try to confuse the attacking team.


Zonal defensive strategies:

In a zone defense rather than marking just one player you mark areas or zones on the field and it is your responsiblilty to protect that area.
 
There are many different ways of playing zones but they are usually used when it is windy as you generally allow short sideways and backwards passes but block upfield passes.   You hope to cause errors by the opposing handlers making them throw lots of cross field passes.  
Up field throws are pressured into being over the top and you hope to get into challenge for the disc.

Communication is a vital component of this defence as positioning is very important and to get it right everyone must work together.

The sideline is very helpful too, providing on field players with information on the location of offensive players and their defensive position.



Cup:

Not played that often anymore as the next zone "Junk" is usually prefered.
It is effective against teams with weak handlers but most teams have such good disc skills that they beat this zone fairly easily.  
The cup is basically an extended force, usually towards the middle of the field, due to the 3m double team rule, 1 player forces and the next 2 look to extend this force from 3m away.
The other 4 downfield players look to mark an area each tracking offensive players who come into their area.