Those are <p>, or paragraph tags. You can give that tag attributes such as border, size, colour, positioning, etc.
The curved corners you see are images (or sometimes 2) merged into one, thus giving that curved look.
If you put the two together, you'll get the effect you see. Think of it like a jigsaw puzzle. When you write the code, you "tell" the browser where you want it to be or how to be displayed (see attributes I mentioned above).