Show or hide hidden files in Mac OS

To show hidden files and folders in Finder, open a terminal window and type this:
defaults write com.apple.finder AppleShowAllFiles TRUE

(the above sentence is all in one line), press return, and then type:
killall Finder (and press return) 

Finder will relaunch and you will be able to see hidden files and folders.

To revert (hide hidden items), type this in terminal:
defaults write com.apple.finder AppleShowAllFiles FALSE

(all in one line again), press return, and then this:
killall Finder (and press return)

That's it!