What is a set of chromosome and what is a pair of chromosome
Yes those two expressions refer to different things.

Set of chromosome

The term "set of chromosome" refers to ploidy number. A haploid has one set of chromosome, a diploid has two sets of chromosomes, an hexaploid has six sets of chromosomes.

In humans, each set of chromosome is made of 23 chromosomes (22 autosomes and 1 sex chromosome).

Pair of chromosome

A pair of chromosome refers to the two homologous chromosomes in a diploid individual (one chromosome from each set for a given chromosome number).

Source of information

If the above is unclear, then you should have a look at any intro course to genetic such as these courses by Khan Academy for example.