There is a very small air resistance at the altitude of the ISS. This drag causes the ISS to lose up to 5 cm/s (0.1 mph) of velocity and 100 meters (330 ft) of altitude each day.
To compensate for this, about once a month the ISS fires its thrusters to increase its altitude. This maneuver is called a reboost, it is done by modules at the rear of the International Space Station (ISS), such as the Progress, ATV (pictured below), or if necessary the Service Module, itself.
There are two types of reboost - single burn and two-burn. A single burn reboost involves one firing of the thrusters. The impact of the firing is an increase in altitude on the opposite side of the planet. This type of reboost is done for small reboosts because it does change the eccentricity of the orbit.
The general idea is that if we create a delta-v at a point, that delta-v will affect the vehicle throughout its orbit. So in the above picture we can see that for the first half of the orbit (moving counterclockwise) it lifts the ISS away from its nominal orbit. But once we pass the 180 degree point, we can see that the delta-v now lowers the ISS back to its original point.
A two burn reboost essentially starts like a single burn reboost, but at the 180 degree point it fires the thrusters again to cancel out the original delta-v. This results in the ISS being in a new circular orbit at the altitude of the second burn.
The design envelope of the ISS is to keep it between 280 km and 460 km. But we don't usually reboost up to 460 km and then drift down to 280 km. The reason for that is that we don't want to make the visiting vehicles work so hard and burn so much fuel to get up to 460 km.
So, that means we do smaller, more frequent reboosts. They occur about once a month and involve a delta-v of about 2m/s and involves firing thrusters for about 900 seconds, although that is variable depending on which module does the burn.
Most often, reboosts are done by an attached Progress module. We usually use the smaller thrusters on the vehicle, because we do not want the acceleration on the vehicle to greatly affect ongoing payload science. Typically we use four thrusters that each have a force of 13.3 kg-f (29.3 lbf).