Beauty is relative. Meaning that perception of beauty differs from one person to another. What you find beautiful, others may not. It's just like having different tastes on food, clothes, shoes etc. When you say something is beautiful, it is simply because you find it beautiful.
Some people may equate beauty to physical appearance. One very common perception is that having fair skin, petite figure or even average height is beautiful. This is where the society and media play a huge role. They tend to condition people's minds that there is really a concrete and absolute definition of beauty, when in fact there isn't. Nonetheless, others may still not have similar judgment.
Others may look past physical appearance and consider a person's character when they define beauty. Personally, this is what I think is more valuable.