Weirdly, we won't know the official answer to "what is autism and what isn't?" until 2013 when there's the next publication of the "DSM V", which guides a lot of the world's thinking on autism and other conditions.
This is my understanding of what I read:
At the minute, the experts are thinking that there's actually no difference between autism and Asperger syndrome at all because autism is social communication difficulties plus huge need for routine/predictability.
It can ALSO occur with separate other diagnoses - e.g. speech delay/absence of speech, brain injury, learning disability, epilepsy, major sensory issues, ADHD, etc.
But they are thinking that none of those things are autism - they're other things.
So...if a child comes along for a diagnosis, they might get a far more complicated but maybe more useful one that says (in layman's terms)
e.g.
Mild autism, moderate speech delay, moderate epilepsy, very low IQ
Severe autism, no speech delay, severe sensory issues
Moderate autism, moderate dyspraxia, severe sensory issues
etc etc. And then the idea is that you'd get a package of care that looks at ALL the bits of this and actually helps the whole child, not just a bit of them.
Er, that's the theory, so far.
But we have to wait patiently until 2013 to see what happens. Meantime they'll continue to diagnose on the old standards.