Bubbles attempts to turn your web browser into a place where you can write just as you would on real paper -- whether that means typing a letter, making a collage, sketching notes, or a combination of the three. It provides several tools that allow you to create your own, personalised letters and easily share them with others.
Bubbles works by providing you with a blank sheet of paper and tools you'd find in a word processor and in some art applications. You can type, but you can also upload images, draw with a pen or pencil, create shapes, and more. This is practical if you want to draw something in a letter you want to send someone or just fun if you want to sketch something in some notes you're taking. Bubbles was created with social capabilities, so you can easily share what you create with a link -- such as the example you see above.
Although Bubbles essentially wants to be digital paper, one of the (probably obvious) drawbacks is that your mouse or trackpad isn't exactly the best surface, just as your hand or finger isn't a great replacement for the pen. You can digital sign a letter, but how good that signature is going to look is another story. If you have a drawing tablet, or even a capacitive stylus and multitouch trackpad, you'll be in better shape.
Everyone else will struggle to actually sketch like the webapp suggests. Additionally, I couldn't find a way to make text wrap around images. Doing that manually is pretty tedious, so hopefully they'll add that feature. All of that said, Bubbles is a pretty comprehensive word processor. While your hardware may not be capable of taking full advantage of its digital paper aspirations, Bubbles is mostly up to the task if you are.