A Primary Button is a major design element in you have a single button on a line in your app/website. It is useful for call-to-action buttons. The button can have 3 components (an icon, button text, and another line of text called sub text), all are optional, but it makes sense to have at least one. Here are some examples of buttons:
Button with all 3 elements.
A button without an icon:
A button without button text, but with sub text:
A button with just an icon:
You have a large amount of control over the button including size, color, padding, border, and shadowing.
Buttons have a link (links are optional) that you can use to do the usual MyPhotoApp linking capabilities.