The Alexandrian Tradition was created in the 1960s by Alex Sanders and his wife Maxine Sanders. Though Alex Sanders originally claimed to have been initiated by his grandmother when he was seven, he later admitted that this was false. In actuality, he was initiated into a Gardnerian coven.
Alex Sanders decided to publicize Wicca, but encountered opposition from other members of the Craft. But his publicity allowed him to initiate a great deal of people, including Stewart Farrar and Janet Owen. Stewart and Janet would later marry and publish many books about Wicca.
The similarities between Alexandrian Wicca and Gardnerian Wicca are large and the topic of more than a few neopagan in-jokes.
Alexandrian Wicca, like Gardnerian, has a hierarchical structure and an emphasis upon training in ceremonial magic topics.
Also like Gardnerian Wicca, they have a 3-degree system. The tools and rituals are similar, but not identical. Supposedly, there are some fundamental philosophical differences as well.
The Alexandrian tradition generally meets weekly, or at least on full moons, new moons, and the traditional festivals.
They often will allow non-initiates to attend circles.
Thanks to the Covenant of the Goddess for information. To learn more, look at http://www.cog.org/wicca/trads/alex.htm or the book What Witches Do by Stewart Farrar.