what is the difference between use of var_dump() and mage::log() to debug?

Question

I mean when i can use var_dump() and mage::log().

I have some idea about log. if we enable log settings and for example if we use mage::log($obj).it will print $obj information in system.log file.

where as var_dump() print direclty in browser.

Please comment if i'm anything wrong.