どれ belongs to the same set of words as これ, それ, and あれ. In Japanese, this group is known as こそあど言葉(ことば). While the other three refer to a particular item, words that begin with ど are used when asking a question about something. どれ is the equivalent of 'which' in English and is used when choosing from three or more items.

• どれが泳(およ)ぎますか。
  Which one of these swims?
• どれが安(やす)いですか？
  Which one is inexpensive?
• どれが便利(べんり)ですか？
  Which one is convenient?
• どれが先生(せんせい)のかばんですか？
  Which one is the teacher's bag?

Based on these sentences, you can see that どれ is frequently followed by the particle が, particularly when it functions as the subject of a question. However, if the topic is already established with a noun, は will be used instead.

• 田中(たなか)さんの傘(かさ)はどれ？
  Which one is Tanaka's umbrella?

When you are only presented with two options, どちら + の or the more casual どっち + の is used in place of どれ. This is a distinction that has become standard in modern Japanese, though these words were not always used this way historically.

• どちらの本(ほん)を読(よ)みますか？
  Which book will you read? (Out of these two choices)
• どっちの映画(えいが)が面白(おもしろ)い？
  Which movie is more interesting? (Out of these two choices)