Technically they provide the same service but they have different ways of going about it, have different communities, and have different agendas (Shelfari, another book social network, was bought by Amazon a few years back, while Goodreads started purging its Amazon entries last year).
There's also an article on Book Riot that differentiates the two websites:
http://bookriot.com/2012/07/31/goodr...hing-part-one/