Almond extract is mostly alcohol and water plus a flavoring compound called benzaldehyde, which gives it its strong almond taste.
Pure almond extract gets its benzaldehyde from bitter almond oil, often pressed from almond, apricot, or peach kernels, with the toxic hydrogen cyanide removed. Imitation almond extract uses synthetic benzaldehyde instead, and tastes very similar in baked goods. Check the label for the exact base, since some brands add sugar or other flavors.
Source: Benzaldehyde, Wikipedia