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?