A seamstress or seamster is a sewing professional who primarily makes clothes using a design or pattern. They also repair garments and make alterations to clothing. A seamstress works on all types of garments including men's and women's clothing and non-clothing items like curtains and pillows.
A tailor is a professional who primarily alters clothing for a customer. They hem and mend garments but can sometimes make clothes using a design or pattern. A tailor tends to work more with men's suits and coats but may also work on women's pieces including fancy dresses and ball gowns.
Both seamstresses, seamsters, and tailors work in dry cleaners, alteration establishments, and high-end fashion stores. In a bridal boutique, you are likely to work with a seamstress. In a men's suit store, you are more likely to work with a tailor.