What is Diceware?
Diceware is a way to make a passphrase with ordinary dice. Five rolls give a number from 11111 to 66666, and each number maps to one word in a list of 7,776. Because the dice choose, not you and not a computer, nobody can predict the result.
How many words?
Each word adds about 12.9 bits of randomness. Five words give about 65 bits and six give about 78, which makes a strong master password. See how to make one memorable.
Credit
Words come from the Electronic Frontier Foundation's large word list, published under a Creative Commons Attribution licence.