A seamstress is a person whose job involves sewing clothing. You could be a seamstress if you hem your own pants, but most seamstresses work in factories sewing garments using sewing machines.
Traditionally, a seamstress was a woman who sewed seams in clothes using a machine, or occasionally by hand. Seamstresses weren't considered as skilled as a dressmaker, who makes custom clothing, or a tailor, who alters clothes to fit a specific person. Each of these titles is a distinct job, but they are commonly used interchangeably. Some people use the newer, unisex term sewist instead of seamstress.