Classic example of resonance - Tacoma Narrows Bridge collapse

This is the bridge that, due to winds hitting at just the right speed and angle, caused vibrations within the superstructure of the bridge at the resonance frequency.  Resonance is when an object vibrates at its natural frequency, such as a tuning fork has a natural frequency.

Another example of resonance is pushing someone on a swing.  Your push needs to be in synch with the natural frequency of the swing (i.e. a pendulum) if you want to maximize the amplitude of the swing.  If your push frequency is out of phase with the natural frequency of the swing, you will at times be 'working against' each other, and not achieve maximum amplitude.

Here is another example of a so-called 'wolf tone' on a string instrument, where a short piece of a string resonates with the full length of the string. Yet another is the Schumann resonance in the ionosphere each time there is a lightning bolt, and one last one is when a singer can find and hold a note that will make a wine glass resonate and explode! See the second video below to see the wine glass in slow motion.