Every noun in Spanish is considered to have either masculine or feminine gender for grammatical purposes.
Many Spanish adjectives and determiners alter their form to agree in gender with the nouns that they modify, and likewise many pronouns show gender agreement with their antecedent nouns. There is no neuter gender for Spanish nouns, but some pronouns are considered to have neuter gender. A few nouns are said to be of "ambiguous" gender, meaning that they are sometimes treated as masculine and sometimes as feminine.
Additionally, the terms "common gender" and "epicene gender" are used to classify ways in which grammatical gender interacts (or not) with "natural gender" (the sex of a person or animal).
Grammatical gender in Spanish must not be equated with sex, although most nouns referring to male persons are grammatically masculine, and most referring to females are feminine. Exceptionally, persona ('person') and víctima ('victim') are always feminine, even when they refer to a male. When referring to non-living objects, there is in most cases no more "sense" to noun genders in Spanish than there is in other gendered languages. The gender of a Spanish noun is in most cases an arbitrary fact about it which must be memorized.
Masculine (masculino): As a general rule, nouns ending in -o (libro 'book', zapato 'shoe') and nouns which refer to males (profesor, padre 'father') are masculine. Exceptionally, mano ('hand') is feminine. Also some colloquial shortened forms of feminine nouns end with -o: la foto(grafía) ('photograph'), la disco(teca) ('discothèque'), la moto(cicleta) ('motorcycle'), la radio(difusión) ('radio [broadcasting]').
Feminine (femenino): As a general rule, nouns ending in -a (casa 'house', boca 'mouth') and nouns which refer to females (madre 'mother', mujer 'woman, wife') are feminine. Similarly, the endings -ción, -sión, -dad, and -tad indicate feminine gender. Exceptionally, día ('day') is masculine. Likewise, nouns of Greek origin ending in -ma (drama, problema) or "-ta" ("planeta", "profeta") are masculine. (These "Greek" nouns can often be identified by their derived adjectives ending in -mático.)
Neuter (neutro): The pronoun ello ('it, the aforementioned concept') and the demonstrative pronouns esto ('this [idea or unnamed thing]'), eso ('that'), and aquello ('that') are said to have neuter gender because they do not have a gendered noun as their antecedent, but rather refer to a whole idea, a clause, or an object that has not been named in the discourse. The neuter article lo (not to be confused with the masculine/neuter object (clitic) pronoun lo), is not used with nouns, since there are no neuter nouns. It is used with adjectives to create abstract "nouns": lo bueno, the good part (of it); lo importante, what is important (about it). Contrast el bueno/la buena, the good person or thing.
Common (común): "Common gender" is the term applied to those nouns, referring to persons, that keep the same form regardless of the sex of the person, but which change their grammatical gender. For example, el violinista ('the male violinist'), la violinista ('the female violinist'), el mártir ('the male martyr'), la mártir ('the female martyr'), el testigo ('the male witness'), la testigo ('the female witness'), el espía ('the male spy'), la espía ('the female spy'), etc. To this gender belong present participles derived from active verbs and used as nouns, such as el estudiante ('the male student'), la estudiante ('the female student'), el atacante ('the male attacker'), la atacante ('the female attacker'), el presidente ('the male president'), la presidente ('the female president' — although la presidenta is also often used), etc.
Epicene (epiceno): "Epicene gender" is the term applied to those nouns that have only one grammatical gender, masculine or feminine, but can refer to a living creature of either sex. Most animal names are of this type. E.g.: el ratón ('mouse'), la rata ('rat'), la rana ('frog'), la comadreja ('weasel'), la liebre ('hare'), la hormiga ('ant'), el búho ('owl'), el escarabajo ('beetle'), el buitre ('vulture'), el delfín ('dolphin'), el cóndor ('condor'), la paloma ('dove'), la llama ('llama'). To specify sex, a modifying word is added, with no change of gender: el delfín macho ('the male dolphin'), el delfín hembra ('the female dolphin'), la comadreja macho, la comadreja hembra (male and female weasles respectively).
Ambiguous (ambiguo): Nouns whose grammatical gender varies in usage are said to be of "ambiguous" gender. Often the change of gender brings about a change of connotation. E.g.: el mar ('the sea'), la mar ('the sea', poetic or among sailors), el calor ('heat'), la calor (archaic), el azúcar, la azúcar ('sugar').
The role of gender in Spanish is treated in great detail (in Spanish) in the newest grammar of the Real Academia Española (Nueva gramática de la lengua española, Madrid, Espasa, 2009, pp. 5–43), and that section happens to be made accessible as an on-line sample of the larger work.
