The aeolian mode is built from the sixth degree of a major scale. It is also called the "natural minor" scale, and is also called the "relative minor" scale to the major scale that it came from.

Example:
C major scale: C D E F G A B C
A aeolian mode: A B C D E F G A

A major scale (for comparison to A aeolian):
A B C# D E F# G# A

What's in it:

The aeolian mode has a b3, a b6, and a b7 (C, F, and G, as opposed to the C#, F# and G# in the A major scale). The b3 makes it a minor mode.

Hear the A major scale (A ionian)
Hear the A aeolian mode