Yet usually functions as an adverb. One of its main uses is in final position with negative verbs or questions to indicate that something hasn't happened but with the expectation that it will happen at some point, as in 'I haven't done my homework yet' or 'Have you finished with the newspaper yet?' It is also used with negative verbs, as in 'It's only 7 o'clock - we don't need to be at the airport yet', to mean 'so early'. Yet is often used with superlatives to mean 'so far', as in 'This was Ronaldo's best game yet' or 'That was our worst holiday experience yet'.
Apart from its function as an adverb, yet can also be a conjunction, where it means 'but'. Examples are fixed expressions such as 'so near yet so far' (where it cannot be replaced by but) and those more usually found in written English such as 'She was calm yet determined' (where it can be replaced by but).
As an adverb, yet is also often used together with 'again', as in 'The naughty dog's run away yet again!' to indicate exasperation. This expression can also be used in initial position for extra effect, e.g. 'Yet again you have exceeded my worst expectations!' Used with 'another' it indicates surprise, either in a positive sense, as in 'This was yet another excellent performance by Barcelona', or in a negative sense, as in 'This was yet another dreadful performance by United'.