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?