Whats a sonnet, i know its a type of poem but what is it? please help me thanks.?

Sonnets usually are about idealised objects of love, for example the unobtainable woman who is idolised and put on a pedestal by the poet/narrator. However Shakespeare was a little different, in his sonnets though he does idolise the young man he is more a obtainable target, as for the sonnets about the dark lady, he uses the sonnets to call her names, tell her she is ugly, say she is a but despite all her failings he loves her, for examples of these see, 129, 130, 135 and 144. It is also quite telling that he wrote 126 sonnets to the fair youth and only 26 to the dark lady, tho some to the youth feature the dark lady. HTH.