nkf stands for "Network Kanji Filter," and it's a program used to convert between various encodings used for Japanese character sets. It handles ISO-2022-JP, Shift_JIS, EUC-JP, UTF-8, and some variants of the above (such as the Microsoft version of Shift_JIS). As well, it can do a limited amount of MIME decoding. It also can try to guess the input encoding so that you don't have to specify it.