A pro-drop language is a language which allows pronouns to be omitted when they can be inferred. In pro-drop languages, the use of the subject pronoun is considered unnecessary. However, languages such as Japanese and Korean also allow the deletion of other classes of pronouns. Languages which only allow the deletion of subject pronouns can be classified as partially pro-drop languages.
Besides Japanese and Korean, Chinese, Russian and Hungarian exhibit frequent pro-drop features. Italian and Spanish are classified as partially pro-drop because they only allow the omission of subject pronouns. Finnish doesn't allow the deletion of all subject pronouns. Third person singular and plural subject pronouns are never deleted.
In languages which allow the omission of pronouns, use of the pronoun is often for cases of emphasis and formality. For example, the Spanish sentence Yo no canto bien (I don't sing well) may be considered more formal than No canto bien, which omits the subject pronoun. The use of the pronoun also makes the sentence more emphatic. In response to the question Who doesn't sing well? it's natural to include the subject pronoun in the reply.
Languages which allow the omission of pronouns which can be inferred are pro-drop languages. They include Chinese, Japanese and Korean. Those which don't allow it are non-pro-drop languages. English, French and German belong to this group.