Your stepmother is your father's wife who is otherwise not related to you. Your father divorced or was widowed and got remarried. His new wife is your stepmother.
A mother-in-law is your spouse's mother.
Your mother-in-law can also be your spouse's stepmother.
Example: Mr. Smith's wife dies when his son, John, is 10 years old. Mr. Smith marries a new wife, Jane, two years later. Jane Smith becomes John's stepmother. Ten years later John marries you. Jane is then both a mother-in-law (to you) and a stepmother (to your husband, John).