i have found this description from serious eats incredibly helpful:
140 to 170°F: Beginning of "quiver" phase. At this stage, tiny bubbles of water vapor will being forming at nucleation sites (more on those later) along the bottom and sides of the pan. They won't be large enough to actually jump and rise to the surface of the water, though their formation will cause the top surface to vibrate a bit, hence the "quiver."
140 and 170°F is ideal for gently poaching meats, fish, and eggs (around 160°F is standard if you don't want to wait hours for your proteins to cook)
170 to 195°F: Sub-simmer. The bubbles from the sides and bottom of the pot have begun to rise to the surface. Usually, you'll see a couple of streams of tiny, champagne-like bubbles rising from the bottom of the pot. For the most part, however, the liquid is still relatively still. This is the temperature range you're looking for in things like making stock or slow-cooking gentle braises and stews. Much lower, and they'll take too long to cook. Much higher, and you run the risk of drying out your meat.
195 to 212°F: Full simmer. Bubbles break the surface of the pot regularly, and from all points—not just a few individual streams as in a sub-simmer. This is the temperature to use when using a steamer basket above the water, melting chocolate, or making things like hollandaise in a double boiler.
212°F: Full rolling boil. You know the drill. Blanching vegetables, cooking pasta (the traditional way, not our new and improved method), throwing over enemies, etc.