We get a lot of questions about what Shaolin 'is'. The best place to start is with a list of quick facts below:
Shaolin is a small, liberal sect of Buddhism faith. Theologically, it exists where the schools of Zen (Chan) and Mahayana intersect. Shaolin combines meditation, community service and spiritual enlightenment into its core set of values.
It is practiced independently in many places around the world, but mainly in Canada, the US, Australia, the UK and parts of Europe. It is not associated to the present day monastery/temple in China. Shaolin did not originate in China (but for many centuries it had a very strong presence there). It is not a centralized religious institution with a worldwide central headquarters in China.Parts of Shaolin Buddhism were refined in China after non-Chinese foreigner Indian trained Bodhidharma set up shop there in the 6th century, until the Chinese government of the late 19th century shut it down. Shaolin Buddhist priests, after leaving China more than 100 years ago, primarily immigrated to the US and Canada to continue teaching Shaolin Buddhism, almost entirely in secret.
Shaolin is not another name for the martial art known as Kung Fu. However, Shaolin does practice Shaolin style Kung Fu, which is primarily for spiritual development, not for fighting or violence. As such, Shaolin Kung Fu is a branch of Shaolin Buddhism.
You do not need to convert or stop believing in your current religion--Shaolin allows for overlapping religious beliefs.
A non biased primer in Buddhism via wiki
Zen (Chan) Buddhism -- a general primer in Zen via wiki
A more detailed primer in Mahayana Buddhism
The Sacred Shrine of See Mee Bee, which stands for Shaolin Mahayana Buddhism
An academic explanation: lack of authenticity current Chinese Henan Province Shaolin Monastery