か

か is a particle that frequently comes at the end of sentences in Japanese, and carries the same meaning as a question mark in English. (It is not limited to appearing at the end of sentences, however). In addition to questions, か can also express uncertainty about something, without strictly being a question.

• これはコップですか。
  Is this a cup? (Plain question)
• あなたのアパートは大(おお)きいですか。
  Is your apartment big? (Plain question)
• 美味(おい)しいか分(わ)からない。
  I don't know if this is good or bad. (Expressing uncertainty, but not a question)
• これは何(なに)か分(わ)からない。
  I don't know what this is. (Expressing uncertainty, but not a question)

か regularly comes after the polite form of words, such as ～ですか, ～ますか. However, in casual conversation か can be used directly after the word that the speaker is showing uncertainty about.

• 大丈夫(だいじょうぶ)ですか？
  Are you okay? (Standard pattern)
• 大丈夫(だいじょうぶ)か？
  Are you okay? (Very casual pattern)

Both examples are grammatically correct, but the sentence without です may be considered rude by some people (as it sounds too familiar/direct).