Question

Which two things are being compared in the poem? harbors and cities  fog and cats  cat feet and harbors fog and moving 

Answer 1

From what I can make out, I think the two things being compared are the fog of the harbour, and the city. 

Answer 2

Fog is a short poem by Carl Sandburg:

The fog comes

on little cat feet.

It sits looking

over harbor and city

on silent haunches

and then moves on.

Sandburg tries to describe similarities between something alive and not alive, correspondingly between cat and fog. He tries to show that fog moves like a cat.