A target that has more than half cover or almost-complete concealment gets a +3 bonud to their Dodge score. A target that has almost complete cover has a +5 bonus to Dodge. As usual, there are Schticks to mitigate this, and the usual expediency is to simply move around the cover.
In general, cover only applies if it is closer to the target than it is to the attacker. In the case where two people is fighting over cover, the defender gets the bonus until the attacker manages to hit or to make a stunt to bypass the cover.