Sound levels above 85 decibels are dangerous to your hearing and can cause irreversible damage. Amplified music at about 110 decibels can damage your ears after four to 30 minutes of exposure.
Repeated exposure to loud music will damage the tiny hairs inside the cochlea of your ear, which turn sound into nerve impulses that are then processed by your brain. This damage is reversible unless it is due to repetitive and prolonged exposure to loud music.
Earbud-style headphones are dangerous because they don't block outside noise as well as headphones that fit over the ear. This results in you listening at volume levels that can damage your hearing in as little as an hour and a half.
Symptoms leading up to and including possible damage include tinnitus (ringing in the ears), hearing sound as if it's being muffled, and a difficult time hearing and understanding speech in loud environments.
Prevent hearing loss by using ear protection such as ear plugs when attending loud musical events. Keep your headphones at a level where you can still hear normal conversation above your music.