Well, here's a breakdown of basic latte art designs...
A simple heart is just a blob that is poured and then pulled into a heart shape. It takes some skill to get some really good definition or shading into a heart. The heart here has a bit of that, along with a mini-wave that was poured first.
The tulip consists of two or more hearts that are stacked one on top of the other. (The end result may or may not look like a real-life tulip.)
A rosetta is a leaf-like pattern made by rocking a pour back and fourth to draw the leaves and then pulling through the middle to finish off with the stem. A double rosetta would be two of these in one cup. Baristas have been known to pull off triple or quadruple rosettas. The most I've seen in one cup is 7.
I've basically been striving to do the perfect rosetta my whole barista career. It's a design that is fairly easy to pull off but difficult to master.
Etching is done when some kind of tool is used to "draw" in the milk and crema after the drink is poured. I, like most baristas, use the tip of a milk thermometer to draw with. Designs are drawn by dragging the colors in, out and around, or by picking up a bit of one color (like loading a paintbrush) and drawing in the other. The tip of the tool is usually wiped off with a cloth between strokes.
While many latte art purists scoff at the idea of etching, I can guarantee that I've wowed more customers with easy etching than with elaborate free-pour latte art.
Etching with Syrup
It's also possible to use a bit of chocolate drizzle to make some etching designs pop. I'll only do this in customers' drinks that actually have some chocolate already in them (like mochas or hot chocolates). Sometimes you can also use syrups that have other colors - I've used raspberry or caramel quite nicely. Etching with syrup is also possible even if you don't have any contrasting crema and milk (if it's just completely white or brown), so I've pulled out the drizzle bottle at times when an attempted latte art pour has gone bad.