What is this macbook keyboard popup bubble? While I am typing, my macbook randomly (to me anyway) pops up a speech bubble with alternative characters. I can be flying on typing the rest of the sentence before I realise a number of characters have not typed. I do not understand what triggers this to appear. Does anyone know what this bubble is and how to turn it off? 

It must be some accessibility feature? But I cannot see what the setting is and how to turn it off?! Its driving me crazy!!



Update:

OK, so thanks to replies I know now it is the mac accent character picker. But I'm not holding down a key when it appears, I'm typing normally without pause. Before I turn it off completely as suggested, is there any sensitivity setting I can check to confirm how long it waits for a key to be held down?

If not, I guess it might indicate a hardware problem with my keyboard then and turning it off completely is the only option.
This is the character picker for entering accented characters. This appears when you hold down a key on the keyboard. This can be disabled globally using defaults:

defaults write -g ApplePressAndHoldEnabled -bool false