Both words are normally used to refer to the pleasing appearance of the face — usually that of women. Beautiful is the stronger word of the two. When you say that something is “beautiful” what you mean is that it is very close to perfection — at least according to the person looking at the object. Remember, beauty lies in the eye of the beholder. Beauty is subjective, what is considered beautiful in one culture may not be considered beautiful in another. One can use the word “beautiful” to talk about things that can be seen. For example, you can refer a woman’s face as being beautiful, a house as being beautiful, and a scene as being beautiful. In fact, you can even use the word with things that cannot be seen. For example, you can talk about a “beautiful plan” or a “beautiful smell”.
The word “pretty”, on the other hand, is not as complimentary as beautiful. When you refer to a woman as being “pretty” what you mean is that she is attractive; she is graceful and full of life, but lacks the perfection. She doesn’t have the qualities to make her beautiful. Pretty is seldom used with people or objects that are big; the word is usually restricted to objects that are small. You cannot refer a 30-storey building as being a “pretty building”. You can talk about a “pretty house” or “pretty shoes”, but not a pretty building. Similarly, a woman who is big built can be “beautiful”, but not “pretty”. The word “pretty” suggests that the person or object is pleasant to look at, but not necessarily impressive.