Well, ask any three witches and you’ll get five to seven answers.
Here’s what I tell folks at work. Ever hear of “Native American” stuff? Mostly people nod and say yes. Well this is Native European. Phrasing it this way allows me to avoid (or maybe just postpone) using the “W” word, and so avoid freaking them out.
Want more detail than that?
What it boils down to is that a guy named Gerald Gardner was trained in a spiritual discipline that he believed went back in some form or other to the stone age. Native British. He seems to have called it “the Craft” or “Wicca” or “the cult” or “Witchcraft”, and he didn’t seem to worry about what exact terminology he used. After he died, folks who didn’t care for what he did started calling his students “Gardnerians”, which they meant as an insult. The name stuck. There are many types and branches of “Wicca” (or any of the above terms); the name “Gardnerian” is reserved for those who were properly taught and initiated by someone who was properly taught and initiated (and so on and so on) all the way back to Gardner.
In general, the British traditions have in common that they have direct connection with deity (as opposed to “going to service” and having a priest tell you what HE thinks that God thinks you should do), and that they practice in groups. Some groups are local gatherings of people who work pretty consistently with each other (called covens) which tend to be scheduled according to the phases of the moon and sometimes there are larger gatherings of multiple covens, which tend to be scheduled according to what Margaret Murray would call “the Sabbats”.