Let's look at an example:
Manchester United are playing Liverpool. A bookmaker will offer odds on either side to win or for the match to end in a draw. When you bet on Manchester United, if they win the match, you win the bet.
If the match ends in either a draw or a Liverpool victory, you lose the bet.
Head to head betting that involves the draw as a third outcome option is also known as 3-way betting. Another sport where 3-way betting is popular is Ice hockey.
Most sports however, only have two possible outcomes. In sports such as tennis, basketball and baseball, you bet on either team to win the match. If the Los Angeles Lakers are playing the Boston Celtics, you can bet that either the Lakers will win or the Celtics will win.
As you can see, much of the popularity of head to head betting is derived from its simplistic format. It's an easy and very familiar way for people to bet and an ideal starting point for anybody beginning their journey into sports betting.