Cats are special in their own way - they can be very affectionate and 'sticky', but they are on the whole very independent and exhibit periods of aloofness and detachment from their human owners.
Dogs on the other hand form stronger bonds (generally) with humans - they're loyal, affectionate and are always there for you. Some of them have almost a 'human' personality in the way they communicate and interact with humans. The more intelligent breeds can be trained to perform various tasks. Most dogs will defend their masters to the death and mourn for long periods when their masters die.