Life seemed much simpler when the hardware store carried only three kinds of caulk. Now, home centers dedicate an entire aisle to caulk. As a result, confusion reigns when it's time to do a simple job.
But I don't think caulking needs to be complicated. Other than specialized caulk for specific tasks, like patching gutters, I've found that a high-quality siliconized acrylic-latex caulk is an excellent choice for almost all interior and exterior uses.
I know people who have paid $5 to $6 per tube for 100% silicone caulk and filled every crevice and seam they could find. It was only at the end of the job that they discovered that paint won't stick to silicone caulk. They were left with a job that could have looked much better.
Acrylic-latex caulk can be painted, cleans up with water and is almost half the price of 100% silicone. But you should use silicone tub-and-tile caulk in the bathroom because it resists mildew and doesn't shrink.
For gaps of a quarter-inch or more, or where the caulk must join dissimilar materials, use urethane caulk because it's more elastic. For joints more than 3/8-inch wide, pack the crack with foam backer rod before applying caulk so the seam won't crack later. And bridge gaps up to an inch with a combination of backer rod and urethane caulk, but don't try to caulk anything wider than that.
When applying caulk, more is not always better. If you apply too much caulk to a joint and then smooth it out with a finger, the caulk tends to spread onto adjoining surfaces. This is just plain unattractive. And outdoors, this thin layer of caulk will weather differently than the rest of the seam.
Getting an even bead of caulk is easier with a high-quality caulk gun, so skip the 99-cent special. Just remember to cut off the plastic tip at a 45-degree angle, which will help you apply the caulk evenly.
Another tip: Take advantage of the many colors that caulk comes in.