Clock Skew

Clock Skew: 

It is the difference in arrival times of the capture edge at two adjacent Flip-flop pairs.

Positive skew

 If capture clock comes late than launch clock then it is called positive skew.

Negative skew

If capture clock comes early than launch clock it is called –ve skew.

Local skew-

 It is the difference in arrival of clock at two consecutive pins of a sequential element. 

Global skew-

It is Defined as the difference between max insertion delay and the min insertion delay of any flops. 
It is also defined as the difference between shortest clock path delay and longest clock path delay reaching two sequential elements

Boundary skew-

It is defined as the difference between max insertion delay and the min insertion delay of boundary flops.

Useful skew-

If clock is skewed intentionally to resolve violations, it is called useful skew.