python - Difference between stack/unstack, shape and pivot in Pandas -

I am very new to python and I am starting to read on large data analysis using punds.

I have been given the task of getting long form in long form in long form

  choosing '' Pend Woolwars sparse checkers friendly 7 0 22.222222 11.111111 44.444444 Nain Nain 1 8.333333 5.555556 8.333333 11.111111 33.333333 2 8.982036 My initial idea was that I should use the reassignment, but eventually I sat down using the  df.stack () . 
 
 This puts the question in my mind at the forefront: What is the difference between stacking / unstacking, refatting and pivoting? 
 

Which situation should be used?