The most popular Unders/Overs goal market is the 2.5 goal market. Although you can bet on under/over 1.5, 3.5 and 4.5 goals. What often confuses people who are new to football betting is the ‘.5' element of the bet, how can you score half a goal? Of course, you can't but it does allow the bookmaker to draw a metaphorical line in the sand. Alternatively, the bookmaker could just call this the ‘match to have more than 2 goals bet' or ‘match to have 2 goals or less bet' but that doesn't sound very catchy.
For example: if you select a match and back OVER 2.5 goals the score at the end of the game would need to be 2-1, 3-0, 5-1, 3-2, 10-1 or any other scoreline totalling 3 or more goals for you to win the bet. If the match ends 1-1, 0-0, 1-0, 2-0 you've lost as the total is not more than 2 goals.
Given that the typical game of football averages somewhere between 2.4 and 2.6 goals per game (factoring in which league and competition it is) both over 2.5 goals and under 2.5 goals should be available to back generally around the evens mark and more often than not this is the case.