Victorian society was shocked when Sigmund Freud said that all human endeavor is motivated by sex and death. Libido is to sex/creation/life/growth as Destrudo is to pain/breakage/death/corruption.
Animals exercise libido pretty directly in mating and play, whereas humans can (and sometimes do) sublimate the libido into acts of artistic, scientific, philosophical, or social creativity.
The Sponsor™-approved outlets for libido are work, acquisition, and sex (gotta keep 'em classically conditioned).

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I stand corrected. Thank you, Wigs. =)

Freud's definition is as follows: "Libido is a term used in the theory of instincts for describing the dynamic manifestation of sexuality".

Libido is to Eros as Destrudo is to Thanatos.
The Libido is the manifestation of the Eros, the life instinct, it's counter part is Destrudo, not Thanatos.

libido: sexual drive, subjectively experienced and reported. Hypothetically, in psychoanalytic doctrine, it also means the positive life force of Eros as compared with Thanatos, the death force.

Dictionary of Sexology Project: Main Index