What's the difference you ask?

Button-Up

You’re bound to have one in your wardrobe, it’ll have buttons going all the way to the top of the shirt. 

Button-Down 

But then Polo happened.

The traditional polo kit of the 19th century, was a formal collared shirt worn on horseback as players attempted to skilfully strike the ball (or opponent) with the long side of the mallet head. 

The only glaring issue, was that a player's collar would flap up into his face when a horse increased speed – leading to all sorts of horse pile-ups.

To combat this impediment, the players had taken to stitching small buttons onto their shirts, securing the collar in place.

And born was the b-down'

Because God forbid we remove the collar entirely.

Crucially, it means the collar is able to be fastened down to the shirt. When you arm yourself with the button-down feature, it’ll sharpen the overall look while broadening the shoulders.

So there you have it... Now enjoy the show and play nice gentlemen.