Yes and no are two words for expressing the affirmative and the negative respectively in modern English.
English originally used a four-form system up to and including Early Middle English but Modern English has reduced this to a two-form system consisting of just 'yes' and 'no'. Some languages do not answer yes-no questions with single words meaning 'yes' or 'no'. Welsh and Finnish are among several languages that typically employ echo answers (repeating the verb with either an affirmative or negative form) rather than using words for 'yes' and 'no', though both languages do also have words broadly similar to 'yes' and 'no'. Other languages have systems named two-form, three-form, and four-form systems, depending on how many words for yes and no they employ. Some languages, such as Latin, have no yes-no word systems.