What's the difference between an emoji and an emoticon?

An emoticon is a face built from regular punctuation — like :-) or :-P. An emoji is a single Unicode character that is a picture in its own right — 😀, 😛, 🎉.

Emoticons are rendered the same everywhere (they're just ASCII). Emojis are rendered by each platform's emoji font, so they can look different on different devices.