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, el elefante refers to an elephant 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, which are used under limited circumstances, however.)

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 is a chart that shows 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 adjectives, articles and pronouns. You can hear the Spanish words pronounced in the streaming audio file.
 

If you have two or more nouns that are being described by a single adjective, and they are of mixed genders, the masculine adjective is used.

Example: El carro es caro, the car is expensive. La bicicleta es cara, the bicycle is expensive. El carro y la bicicleta son caros, the car and the bicycle are expensive.