Unlike the Romance languages (such as French, Spanish, and Italian), English has three genders for nouns and pronouns: masculine, feminine, and neuter. 

Generally, the English language uses natural gender rather than grammatical gender -- that is, the gender of a word is usually based on its biology (so there is little need to remember whether a word is masculine or feminine). A noun that refers to something with male sexual organs is masculine, a noun that refers to something with female sexual organs is feminine and most other nouns are neuter by default. 

There was a time when you could use the masculine gender by default when you did not know a person's natural gender, but very few people accept this usage any longer. 

There are, moreover, a few tricky points. First, you may refer to all animals in the neuter gender, or you may refer to them by their natural gender:

Neuter

What a beautiful dog! Does it bite?

Natural Gender

What a beautiful dog! Does she bite?

Second, You usually assign mythical beings (such as gods) to a natural gender, even if you do not believe that the beings have actual sexual organs:

God is great. God is good. Let us thank her for our food.

Finally, people sometimes assign natural gender to inanimate objects, especially if they live or work closely with them. When engineers were mostly men, for example, they tended to refer to large machines in the feminine:

She is a fine ship.

For more information, see the discussion of gender-specific nouns.