The adjective beautiful describes a thing that is pleasing to the senses. Most men think women are beautiful, and most women think babies are beautiful, but if you think you're beautiful, you might be vain.
The word beautiful is closely related to the words beauty and beautify, and you'll notice they all share the same beginning, beau-. Almost anything can be described as beautiful because, as the saying goes, "Beauty is in the eye of the beholder." So while you may think the brown spotted grass frog is unattractive, a scientist who spends all day studying them may think it's beautiful.