Niceware words are chosen at random from a list of 65,536 English language words. Each word has 16 bits of entropy. An 8-word random passphrase has 128 bits of entropy which is strong enough to use for cryptographic keys.
In comparison, words chosen from a standard Diceware list with 7,776 words have ~12.92 bits of entropy per word and therefore require more words to achieve the same security level.
Niceware can also be used to convert existing cryptographic keys and other random bytes into human-readable phrases.