Otherwise know as 'bottom' or more romantically (and more preferable!), 'soul' notes, these are heavier and more tenacious. Base notes are deep, warm and have a powerful influence on the overall perfume blend, so need to be used judiciously and sparingly so as not to overpower the middle and top notes.
Base notes are usually derived from resins, woods, some grasses and leaves. Base notes are integral to a perfume's composition in order to anchor and give depth; fixing and slowing down the evaporation of the other notes.
In natural perfumery base note materials include:
Woods: Sandalwood, Cedarwood
Leaves: Patchouli, Tobacco leaf
Resins/Balsams: Vanilla, Benzoin, Tonka bean, Frankincense
Base notes can be detected in the 'dry down' of a fragrance and can linger for days, depending on the composition of a fragrance and your own skin chemistry.
You will notice base notes feature prominently in Oriental fragrances such as Opium, Shalimar, Poison, Ysatis, Coco and and Chypre-type fragrances such as Ma Griffe (by Carven), Miss Dior, Tabac Blond, Aromatics Elixir (Clinique).