The
correct definition of the Arctic Circle is the northernmost
latitude that features direct
sunlight at sea level in every 24 hour period within a
year. In other words, a point on the Arctic Circle has a sunrise and sunset every day of the year, but just barely on the solstices. It lies at 66 degrees 30 minutes of north latitude.
As one travels away from the Equator, the difference in the amount of daylight between the Winter and Summer solsitces becomes greater. North of the Arctic Circle, there is at least one day where the sun doesn't set at all, and at least one day when it doesn't rise, and by the time one reaches the pole, the sun only rises and sets once a year, a perversely prolonged solstice.
I have been to the Arctic Circle along the Dalton Highway in Alaska. At that point it is slightly above the tree line and passes through the marshy taiga. However, around the world the Circle passes through a variety of climates and terrains. In Greenland and the islands of eastern Arctic Canada, it is well above the tree line and passes through tundra. In western Canada it is below the tree line in places and can actually lie within forest. In Siberia and the Scandanavian countries it passes through both forrested terrain and taiga.
As a constant, winters at the Arctic Circle are ridiculously cold, and summers are brief and can be warm or even hot in certain places like Siberia and Alaska.