What is Art Nude?

What is Art Nude? After the last workshop I did it opened me up to thinking a lot more about the thing I do the most of.. 'Art Nudes' it got me thinking about what it means to me, why I do it why I enjoy it and what makes an image become an 'Art' Nude as opposed to just a nude image?(as a side note you may want t grab a cup of tea and a biscuit this could be a long one.. also worth