If you're an avid bicycler, chances are your brakes are going to need maintenance frequently. Bikes equipped with disc brakes can rub if jostled around from rough trail riding. This is typical with most mountain and cyclocross bikes due to heavy trail use. If your disc brakes rub, this doesn't necessarily mean that you have a serious problem. Chances are the caliper has shifted slightly and is causing the pad to rub against the rotor. This is typically an easy fix for most bikes.
Locate the pad-adjustment bolts on the caliper. Most bikes manufactured today will have this feature. Other bikes will have bolts holding the caliper onto the fork that can be adjusted.
Loosen the bolts by turning them counterclockwise slightly with an adjustable wrench. Bolt diameters will vary between models.
Adjust the pads in or out using the pad-adjustment bolts on the caliper if you have this feature. These will have markings on the caliper showing you which way to rotate the bolt to move the pads in or out.
Adjust the caliper bolts on the fork--if you have this feature--as loose as possible; then position the caliper straight on the rotor and re-tighten the bolts.
Test ride your bike and apply your brakes to ensure that the pads no longer rub.
After a rough ride, turn your bike upside down and rotate the pedals. Pay close attention to the caliper and look to see if it wobbles or is shifted slightly. If so, realign the caliper to avoid having a rubbing brake.
If you adjust your pads too far in--closer to the rotor--this can cause an abrupt stopping action and send you flying off of the bike. If you adjust the pads too far out, this can cause a delayed stopping action and you may have to jump off of the bike to avoid a collision.