Meaning:  か turns the sentence into a question.

• sentence + か。= question
それは何ですか 。What is that? (= question maker)
あれは教会ですか。Is that a church?
 
※By following a sentence か converts a statement into a yes/no question.
さあ、行きましょう。All right, let's go.
さあ、行きましょうか。Well, shall we go?
※か follows a WH-question (Why, when, who, which, etc.).
それは何ですか。What is that?
 
• sentence + か、sentence + か。= . . . . or . . . .?
明日行きましょうか、今日行きましょうか。Shall I go tomorrow or today?
 

Location:  Behind any sentence.

• When one appears behind a plain form, the sentence sounds very informal.
行きますか。Are you going? Shall we go? (formal)
行くか？Are you going? Shall we go? (informal)
それ、いいですか。Is it good? (formal)
それ、いいか。Is it any good?  (informal)

Gender:  Mostly men use the informal variety.

Level:  Formal or informal

Alternative:  Women tend to use の instead.  Go to no (の)

Warning:

※か may follows a WH-question (Why, when, who, which, etc.).
        それは何ですか 。What is that?

※Inside a sentence, か functions as an alternative marker.  Click here to see a particle ka (か) page.