Homologous chromosomes versus sister chromatids.
Answers
Sister chromatids are 2 copies of a single chromosome and so are genetically identical (they also are physically attached to one another).
Homologous chromosomes are 2 physically different chromosomes that have the same genes but are not genetically identical.
I explain in more detail at the bottom.
In the following image, suppose the green single chromosome is the chromosome 1 you received from your mother, and the purple single chromosome is the chromosome 1 you received from your father. They are homologous chromosomes: they have the same set of genes, but their alleles for those genes can be different (since your mother and father are not genetically identical). When DNA is replicated in S phase, each single homologous chromosome gets copied and the two genetically identical copies remain attached to each other. Those two genetically identical copies that are physically attached to each other at the centromere and comprise a single duplicated chromosome, are called sister chromatids.
