Spanish is the fourth most-spoken language in the world. Originating in Spain, and spoken by most residents there, it is also spoken in Mexico and all of Central and South America except Brazil, Guyana and Surinam. Spanish is also spoken in Cuba, Puerto Rico, the Dominican Republic and other Caribbean islands.
Spanish is a first language for many people in the United States, especially in California, Texas, South Florida, and the Southwest. A romance language, Spanish is closely related to particularly Portuguese, to Italian, and to some extent to French. English and Spanish share approximately one third of the same words, although the pronunciation tends to be very different.

Spanish nouns have genders. Most words applied to persons have natural gender: masculine like hombre ("male person") or feminine, like mujer ("female person"). The gender of some animals, things, and some words applied to persons doesn't follow any rule: they have grammatical gender. Casa (house), rana (frog), and visita (visitor) are of the feminine gender. Árbol (tree), sapo (toad) and genio (genius) are of the masculine gender. Invitado, invitada (guest) and mono, mona (monkey) have natural gender. Fortunately, the gender of a noun is usually (but not always) indicated by the last letter of the word; -o indicates masculine nouns, and -a indicates feminine nouns.
Adjectives also have gender and number. Like nouns, -o usually indicates the masculine form of the adjective, and -a indicates the feminine form. Adding an -s at the end of an adjective or noun makes it plural. Adjectives need to match the noun they describe in both gender and number. For example, borracho "drunk", when modifying las mujeres ("the women"), makes las mujeres borrachas.
In this guide, where genders of nouns or adjectives comes up, we use the form "o/a" to differentiate. It should be clear from context when to use the feminine and when to use the masculine form.