What is the difference between homologous chromatids and sister chromatids?

[Erwin]

Homologous chromatids come from two different parents (one maternal, one paternal), while sister chromatids are identical copies of a single chromosome formed after DNA replication.

 

[danisjohn]

Homologous chromatids belong to homologous chromosomes—one from each parent—and carry the same genes but may have different alleles. Sister chromatids are identical copies of a single chromosome, formed during DNA replication, and joined by a centromere. Homologous chromatids separate in meiosis I, while sister chromatids separate in meiosis II.

 

[Lucas]

Sister chromatids are the exact copies of one chromosome, attached at the centromere, which are formed during DNA replication. Homologous chromatids are part of homologous chromosomes: one chromosome of each parent, and may contain different alleles of the same genes. Whereas sister chromatids are genetically identical, homologous chromatids are similar, but not identical.