What is the definition of set of chromosomes? Does it differ from pair of chromosomes?
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.