A summary of the description of yams:
- starchy in taste, like a baking potato
- dry in texture
- the skin a brownish, almost black color
- non-tapered ends
- the flesh is usually off-white
- requires a year of frost-free weather before harvest
- most commonly available in ethnic markets
- Yam Dioscorea Species
- sweet in taste
- more moist in texture
- can have a skin that is thin and smooth
- can have a yellow or orange flesh and can have slightly tapered ends
- requires 100 to 150 days to harvest
- some varieties remain firm when cooked; some soften. It is the soft variety that is often sold as a yam in the US.
- Scientific name: Ipomoea batatas
In conclusion, you are most likely buying sweet potatoes at a typical grocery store, even if they are labeled yams. It seems not all stores adhere to the requirement to include the label "sweet potatoes" with the yam label. Or, I just missed that part when shopping.
An aside: I was doing internet searches for information and came across this picture on flickr that I think is captioned incorrectly. Someone also left a comment saying so, and I found the pictures above from the link in the comment. I believe the commentor is correct in saying the potato on the left in the photo is not a sweet potato but a yam.