The Unicode standard arranges the characters in 17 so-called planes of a bit more than 65,000 codepoints (216 to be precise) each. The Unicode standard has thus theoretically place for 1,114,112 characters. Some planes are still undefined and will be filled at a later date. The most used characters go into the almost full Basic Multilingual Plane.
The second plane contains mostly ancient characters, like Egyptian Hieroglyphs, and graphic symbols, for example Mahjongg tiles or emoticons. Thirdly the Supplementary Ideographic Plane hosts lots of East Asian characters, that didn’t find a place in the Basic Multilingual Plane. The third-to-last Supplementary Special Purpose Plane is almost completely empty and planned to contain non-character codepoints, like control characters, that define the language of a text. The last two planes are special purpose planes. Codepoints defined there are private, that is, they will never be specified by Unicode and can be freely assigned by third-party programs to whatever seems useful.