Skew,Global Skew,Local Skew,Useful Skew

Skew
• The difference in the arrival of clock signal at the clock pin of different flops.
• Three types of skews are defined: Local skew and Global skew and Useful skew.

Local skew
 The difference in the arrival of clock signal at the clock pin of related flops.

Global skew
The difference in the arrival of clock signal at the clock pin of non related flops.

Useful skew
If clock is skewed intentionally to improve setup slack then it is known as useful skew.

Useful skew is a concept of delaying the capturing flip-flop clock path, this approach helps in meeting setup requirement with in the launch and capture timing path. But the hold-requirement has to be met for the design.

Skew can be positive or negative.
 When data and clock are routed in same direction then it is Positive skew.
 When data and clock are routed in opposite then it is negative skew.

ASIC FLOW vs PHYSICAL DESIGN FLOW

ASIC flow is completely from scratch to end flow i.e. It includes market research, Architecture specification ,RTL coding  and Logic synthesis and other many more phases.

On otherhand  Physical Design flow in simple terms is Netlist to GDS II(Graphic Database System) flow.
Below figure will help you to understand the difference clearly.