Chipotle chiles are not a unique chile, but are actually dried, smoked jalapeno chiles. They are dark brown and wrinkly, in sharp contrast to the glossy look of fresh chiles, but have a wonderfully rich flavor. Most of the heat of the jalapeno is retained when the chile is dried, so chipotles are fairly spicy, falling in the middle ranges of the heat scale. They have smoky overtones, but the real flavor of the chiles is complex and earthy. Instead of causing a sharp and immediate heat, the heat grows gradually as you eat it and takes time to fade away.
In truth, most kinds of chilies can be smoke-dried and sometimes they are all called "chipotle" no matter what they started out as, but the vast majority of the chipotle chiles on the market were originally jalapenos.