Gardnerian Wicca is named for the religion's founder, Gerald Gardner, and it is considered by many people to the be the "original" form of this spiritual path.
Within Wicca as we know it today, there are many different branches that are known as "traditions". It's a similar concept to the denominations of Christianity. In this case, it's the main tradition that Gardner himself established that is still practiced today.
Many of the main tenets of Wicca have already been outlined on other pages (try the Wicca for Beginners page for an overview). Here are a few of the details that make Gardnerian Wicca unique.
Firstly, it is a true lineage tradition. That means you are not really in a Gardnerian coven unless your High Priest or Priestess can trace their training through others back to Gardner himself. The true intimate details of this tradition will not be found in books or on websites, and it can only be learned from others who already know it. Training and study are a big part of Gardnerian Wicca, and that study is broken down into levels. When you reach the 3rd level, you are considered qualified to because priest or priestess of your own coven.
And speaking of covens, yes you have to be in one because of what I just mentioned. Not only because so much information is held within a properly trained coven, but because the family-like bonds between coven-mates is an integral part of the faith.
Other features of Gardnerian Wicca are varied. Groups will practice skyclad for some rituals (naked), and there is a strong influence on male/female duality, fertility and sexuality. The eight Sabbats are honored, and the Wiccan Rede is also very important as a moral guide. Concepts of karma and reincarnation are also central to Gardnerian beliefs.
Large pantheons of Gods and Goddesses are not part of Gardnerian worship like it can be with more eclectic forms of Wicca. There is a single God and a single Goddess, representing the male and female forces in nature. She is seen as the Triple Moon Goddess, and He is represented by the Horned God of the Hunt.
As I am not personally Gardnerian and don't have a view into the inner workings of this Wiccan tradition, I am only basing my information on what I have read about from others in this trad. If you really want to know more about how their version of Wicca works, you'll have to join a coven and learn through practice.
Alexandrian Wicca is very similar to Gardnerian, but that's a topic for another page.