Another name for this holiday is Vaisakha in Sanskrit (Vesak, Waisak, etc.), which is the name of the second month in the Buddhist calendar.
Although it is called the "birthday" of the Buddha, it celebrates not only the birth of the Buddha, but also his enlightenment (Nirvana) and also his passing away. You can read about different practices associated with the holiday at Vesak article in wikipedia.
For a general introduction to the life and teachings of Gautama Buddha, visit the Gautama Buddha article at wikipedia, where you will also find many related links.
Gautama Buddha was born, experienced his enlightenment and passed on in India, making it the historical home of Buddhism. Over time, Buddhism spread throughout the world, although it was not widely practiced in India. Yet it is very important to note that Gautama Buddha is regarded in Hinduism as an avatar of the Hindu god Vishnu. You can read about the Buddha as an avatar of Vishnu at wikipedia. You can also read an article about the Buddha and Hinduism at About.com.
If you would like to celebrate the Buddha Jayanti with a quick introduction to the core ideas of Buddhism, check out the articles about the Four Noble Truths of Buddhism, and the Noble Eightfold Path.
I feel a great personal debt to Cheri Huber, the author of many beautiful books of Buddhist advice and teachings. If you are looking for a very friendly and warm introduction to some Buddhist ways of being, you might want to check out Cheri Huber's website, which contains many of her writings that you can read online.
The Dalai Lama has a website, too!
There is so much absolutely beautiful Buddhist art... making it hard to choose an image for today's post! Here is an image I've used at my World Literature class - a Bodhisattva from 10th-century Nepal: