When you place bets on BetBTC you can opt between take the available odds or place your bet at your own odds, placing your bet unmatched.
When you take other user's unmatched bets you are being the Bet Taker.
When you fill the market, placing your bet at any odd that doesn't make your bet being instantly matched, you are being the Bet Maker.
Commission Charges vary with the type of Bets you place, check the "What is the Fee to use BetBTC?" for more information about fees