|image from Sterlic/Flickr|
So, BOILING, as you already know (because you read that post, right?!) is huge, rolling bubbles, and when you stir, they don't disappear or anything of the type. The water is 212 or above if you're at sea level, but can vary depending on altitude or if you salted the water. While most people are familiar with boiling, it's often too violent for many forms of cooking and can destroy veggies, tear apart pasta and break the crap out of eggs. In general, Boil is a big bully who beats up innocent nommables.
Now, SIMMERING is where you see those little bubbles in the pot, rising slowly, but the water isn't rolling or anything. The temperature is somewhere between 185-200 or so. Most stuff says "Bring the water to a boil, then lower to simmering." Why? Beats me, but I'm assuming they're telling you to just get it hotter faster. If I'm wrong, someone please tell me. Anyway, simmering doesn't beat things up. It's nicer than the Bully Boil, and much more appropriate of a guest for most meals. Simmering is good at taking the time on soups or sauces, and also sweet talking tough meats down to their most gentle side, but if you stick a lid on him, be aware it can get him angry and up to being a Bully Boil. So just watch the temperature.
POACHING sounds like a big meanie, going and stealing things that aren't his, but the only thing he steals is the softest of foods from the other two rougher types. When you're poaching water, you'll see just the start of the bubbles on the bottom of the pot, and your water will be somewhere around 160 ( high-end latte temperature) and 180. Poaching is ideal for very soft and sensitive foods, like eggs, fruit and things that are at risk of falling apart, like gentle hearts (no seriously) and livers and other nasty inside chunks. Most food needs to be completely covered in the water though, as it's not going to build up enough ambient heat to cook the stuff sticking out the top.
So, when a recipe calls for poaching, simmering, or boiling, you're not gonna mess it up anymore, right? Good. Me either. Usually.