The particle か is commonly placed at the end of a sentence to show that it is a question, much like a question mark in English. (However, its use is not exclusively at the end of sentences). In addition to forming questions, か can also be used to show doubt or wonder about something, without it being a direct question.

• あれは駅(えき)ですか。
  Is that the station over there? (Plain question)
• 田中(たなか)さんの車(くるま)は新(あたら)しいですか。
  Is Mr. Tanaka's car new? (Plain question)
• この服(ふく)が安(やす)いか分(わ)からない。
  I don't know if these clothes are cheap or not. (Expressing uncertainty, but not a question)
• これが誰(だれ)のものか知(し)らない。
  I don't know whose this is. (Expressing uncertainty, but not a question)

か is typically attached to the polite form of words, like in ～ですか or ～ますか. In informal speech, however, か can be added right after the word that the speaker feels uncertain about.

• 寒(さむ)いですか？
  Are you cold? (Standard pattern)
• 寒(さむ)いか？
  You cold? (Very casual pattern)

Both sentences are grammatically sound, but the one lacking です might be seen as impolite by some (as it implies a high degree of familiarity or directness).