Because chipotles are jalapenos that have been smoked/dried.
But what's the difference?
They are just as spicy (in my experience), but have a smokier, deep flavor.
If you're cooking with a dried chili, you can put it straight into a soup or stew to impart a smoky flavor and some heat. You can also boil up a cup or 2 of water and drop the chili in. Let it sit until it's soft then remove, cool, and chop!!
These come in small cans like this and the peppers in a tomato/vinegar/herb sauce. You will end up with quite a few peppers in the can, which can be annoying (because I hate wasting food!!!). I have found a pretty easy solution to this problem. On a small plate, scoop out one pepper with a little sauce into a pile. Make several separate piles on your plate and stick the plate in the freezer for a few hours. When they are frozen, pop the individual pepper balls off and put them in a ziplock baggie and store in the freezer. Then whenever your recipe calls for 1 pepper, you can grab it out of the freezer and not waste a whole can! VOILA!