What's the difference between friends and acquaintances?

People often distinguish between an acquaintance and a friend, holding that the former should be used primarily to refer to someone with whom one is not especially close. Many of the earliest uses of acquaintance were in fact in reference to a person with whom one was very close, but the word is now generally reserved for those who are known only slightly.

Acquaintance is often found paired with nodding. Although nodding acquaintance sounds like it describes a person who is known just enough to nod at, it tends to be used instead to refer to a thing or field with which one has a small amount of knowledge or familiarity (and this is the meaning that the phrase has had since its introduction to the language in the early 19th century).