Clock Skew is the difference in arrival times of the active edges of the clock at any two flip-flops.
Clock skew directly impacts timing. When clock at the capturing flop reaches late than the launching flop, the skew is referred to as Positive Skew. This scenario favors setup timing check while make the hold timing check more critical. However, when clock at the launching flop reaches late than the capturing flop, the skew is referred to as Negative Skew. This scenarios favors hold timing check while makes the setup check more critical. Also see: Local Skew and Global Skew.
