varnish cache keys

[Miles]

Hi,

Can someone confirm the default behaviour of varnish in terms of it's 
cache keys?

By default - and without a vary header - is the cache keyed on hostname 
and url only?  If a vary header is added, then the cache also uses those 
headers to key the request.

Specifically, are cookies are ignored by the cache unless specified in 
the vary header or special behaviour in VCL?

I'm trying to understand why we get a low level of cache hits, and want 
to be certain of the facts.

Thanks,

Miles