Drumlins are hills of sediment (generally a quarter of a mile or more in length) that have been streamlined by glacier flow. Thus, they are often elongated. They often occur together in fields, some with as many as several thousand individuals.
The classic drumlin shapes is a hill that highest on its up-glacier end and tapers gently from there, like a half-buried egg. Not all drumlins taper like this, though.
Because they have been streamlined by flowing ice, scientists often use them to understand past glacier flow directions. They are longest in the direction of flow.