Espresso vs. Espresso ristretto
Here, you can see that the standard espresso shot on the left comes up slightly higher in the cup than the ristretto shot to its right. As a general rule, brewing an espresso with less water will lead to a stronger, more concentrated flavor in the cup, with a noticeably heavier mouthfeel.
Cortado vs. Macchiato
In its European home, the cortado is usually a one-to-one ratio of coffee and lightly textured steamed milk, served in a glass without a handle (which requires the drink be a bit cooler than one that would be served in a mug with a handle). A macchiato, on the other hand, is normally presented as a simple shot of espresso "marked" or "stained" with texturized steamed milk—anywhere from a dollop of the stuff to a slightly fuller one-to-one-ratio type pour.
Most cafes have their own interpretation of these drinks, so it's always best to ask. I've found a cortado to often be described as a "mini latte," while an espresso macchiato is more like a "mini cappuccino." Which leads us to the next pair…
Cappuccino vs. Latte
It can be easier to remember the difference if you recall that latte simply means "milk" in Italian, thereby implying that it should have more of a milky-sweet flavor and a slightly more liquid texture, as opposed to its pillowy cousin, the cappuccino.
Cappuccino vs. Latte, continued
The oft-touted "tradition" is that the cappuccino is a drink created in thirds, meaning that it should be equal parts espresso, steamed milk, and foam. Ideally this ensures that all three components exist in perfect harmony in the cup, and the drinker should have exactly as much foam in her last sip as she did in her first.
Many cafes will serve their caffe lattes in a ceramic mug slightly larger than 6 ounces, to ensure that the mathematical ratio leans a bit on the milk-heavy side.
Flat White vs. Caffe Latte
While definitions vary, for the most part a flat white (at left) is a smaller and stronger version of a Australian and New Zealand-style latte (right). Typically served in a ceramic mug with a handle, it usually clocks in at about 5 to 6 ounces of total liquid: 2 ounces of which are espresso, and the rest comprising lightly texturized steamed milk. In comparison, an Australian and New Zealand–style latte is almost exclusively served in a glass—commonly a 7 or so ounce rocks glass—and also comprises about 2 ounces of espresso and the balance of lightly texturized steamed milk.
The main differences? Volume, for one thing (a latte is an ounce or two larger than a flat white), and glassware for another. As with some classic cocktails, the drinking vessel is practically as important to the beverage as its imbibeable components.