Just as maleness or femaleness is an inherent characteristic of human beings and most animals, so is gender an inherent characteristic of nouns in Spanish. With only a few exceptions, mostly those of occupations such as dentista, the gender of nouns doesn't change with the context, and a noun's gender determines the form of many adjectives that describe it.
Although Spanish nouns are classified as either feminine or masculine, remember that there can be feminine nouns that describe things we think of as masculine, and vice versa. For example, una jirafa, which is feminine in form, refers to a giraffe whether it's male or female. For some people, it might be easier to think of them as simply two classifications rather than giving them a sexual identity. Unlike German and some other Indo-European languages, Spanish has no neuter nouns. (There are a few neuter pronouns, such as lo and ello, that are used under limited circumstances, however, and lo can function as a type of neuter definite article with adjectives to provide the meaning of an abstract noun.)
The basic rule is that masculine nouns go with masculine adjectives and articles, and feminine nouns go with feminine adjectives and articles. (In English, the articles are "a," "an" and "the." Also note that in Spanish many adjectives don't have separate masculine and feminine forms.) And if you use a pronoun to refer to a masculine noun, you use a masculine pronoun; feminine pronouns refer to feminine nouns.
Nouns and adjectives that end in -o (or -os for plural adjectives) generally are masculine, and nouns and adjectives that end in -a (or -as for plural adjectives) generally are feminine, although there are exceptions. For example, cada día means "each day." Día ("day") is a masculine noun; cada ("each") can be either feminine or masculine.
Since you can't always tell by looking at a noun or knowing its meaning whether it's masculine or feminine, most dictionaries use notations (f or m) to indicate the gender. And it's also common in vocabulary lists, such as many of them at this site, to precede words with an el for masculine words and a la for feminine words. (El and la both mean "the.")
Here are examples that show some of the ways a noun's gender affects the usage of other words. Some of the examples may be more understandable once you study the lessons on adjectives, articles and pronouns.
- the man: el hombre (masculine article, masculine noun)
- the woman: la mujer (feminine article, feminine noun)
- a man: un hombre (masculine article, masculine noun)
- a woman: una mujer (feminine article, feminine noun)
- the men: los hombres (masculine article, masculine noun)
- the women: las mujeres (feminine article, feminine noun)
- the fat man: el hombre gordo (masculine adjective, masculine noun)
- the fat woman: la mujer gorda (feminine adjective, feminine noun)
- some men: unos hombres (masculine determiner, masculine noun)
- some women: unas mujeres (feminine determiner, feminine noun)
- He is fat: Él es gordo. (masculine pronoun, masculine adjective)
- She is fat: Ella es gorda. (feminine pronoun, feminine adjective)
- Example: El carro es caro, the car is expensive (masculine noun and adjective). La bicicleta es cara, the bicycle is expensive (feminine noun and adjective). El carro y la bicicleta son caros, the car and the bicycle are expensive (masculine and feminine nouns described by a masculine adjective).