When you take the OG reading and look at the alcohol scale on the hydrometer, it tells you the potential alcohol content if all of the dissolved sugar were to ferment. Since there will be some unfermentable sugars in the wort, when you take the FG reading it will be lower than the first reading but still greater than zero. To get the ABV, you would take the original potential alcohol content minus the final potential alcohol content to get the actual ABV (so if the first reading said 7% and the last said 2%, you have a 5% beer: 7-2=5).
If you measure the gravity instead of potential alcohol, you take the first reading minus the last reading and multiply by 131. So if you have an OG of 1.060 and a FG of 1.015, you would do (1.060 - 1.015) x 131 to get your actual ABV.