1. Spread -
A very wide and "spread" out collar. Most suitable for larger knots on your ties, as in the Windor. Think old school business man, large knot, large ego, little...room to breath.
2. Regular -
The basic collar. Buttons right at the neck and the collar itself lays flat and doesn't button down. Sits well with many types of neckwear and even most suits.
3. Button down -
This helps to dress the shirt down, but still keeps it fresh. Helps to pull off that preppy look.
TIP: pair it with a tie, but leave the buttons unbottoned for a preppy look but a little more rough.
When trying on button downs remember you should be able to breathe when fastening the top bottom. A good rule of thumb, a "two finger" gap. Personally, I always leave that top button undone. I think it just adds a little texture to my outfit but for those needing it to be buttoned, leave room to breath.