Answer
Under Article 1868 of the Civil Code, by the contract of agency a person (the agent) binds himself to render some service or to do something in representation or on behalf of another (the principal), with the consent or authority of the latter. An agency may be express or implied, and may be constituted orally unless the law requires a specific form (Article 1869).
Some acts, however, require a specific form: a special power of attorney is required for the acts enumerated in Article 1878 (for example, to sell or mortgage real property, to make a compromise, or to obligate the principal as a guarantor), and an authority to sell a piece of land must be in writing, otherwise the sale is void (Article 1874). Agency is fiduciary in character and, as a general rule, revocable at the will of the principal.