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).