What's the difference between :: (double colon) and -> (arrow) in PHP?

There are two distinct ways to access methods in PHP, but
what's the
difference?

$response->setParameter('foo',
            'bar');

and

sfConfig::set('foo',
            'bar');

I'm
assuming -> (dash with greater than sign or chevron) is used
for functions for variables, and :: (double colons) is used for
functions for classes. Correct?

Is the
=> assignment operator only used to assign data within an
array? Is this in contrast to the = assignment operator which
is used to instantiate or modify a variable?