Genes showing a high sequence similarity are likely to be functionally related and may share a common evolutionary origin. Similar sequences are typically clustered into gene families of potentially homologous genes.
Homologous genes are derived from the same ancestral gene and are further distinguished by being present across different species (orthologs) or within the same species or lineage genome (paralogs).
Orthologs are genes in different species evolved from a common ancestral gene by a speciation (lineage-splitting) event.
Paralogs are gene copies created by a duplication event within the same genome.
While orthologous genes keep the same function, paralogous genes often develop different functions due to missing selective pressure on one copy of the duplicated gene.