A retreated card is face-down and cannot be interacted with, unless an effect specifically mentions retreated cards. This means: 

• Retreated Enemies do not move or attack. 
• Retreated Enemies do not trigger Surveillance in the encounter phase. 
• During the encounter phase the corporation does not place a Hacking Cube on retreated States in the Network. 
• You cannot Fight or Hack a retreated card.

Clarifications:

• Enemies and States without a retreat value can be retreated through effects. 
• Hope placed on a retreated card prevents it from flipping face up. (See: Hope.)
• As retreated cards are face-down, you are not allowed to look at them.