The most common types of thistles in North America include the Canadian thistle, the sowthistle, the musk thistle, and the bull thistle. Certain other thistles, such as the globe thistle, are sometimes grown as garden plants.

Thistle plants are green. Some are covered with fine white hairs, which make the plant look blue-green. Thistles vary in size and form. Common North American types can grow from about 1 to 8 feet (0.3 to 2.4 meters) in height. Some kinds have branches on their stems, which often have sharp spines. The leaves are typically deeply toothed, or ragged, on the edges, and some types have prickles on the top of the leaves.

The flowers of thistles consist of many tiny florets, or little flowers, growing together in one flower head. Most types of thistles can produce dozens of flower heads on a single plant. The flowers are pink, purple, or yellow in a number of common North American thistles. They range in width from less than 1 inch to about 3 inches (2.5 to 7.5 centimeters). The flowers produce feathery seeds that scatter in the wind. Some seeds have small hooks that attach to animal fur.

Thistles grow mostly from seed, but some types can also sprout plants from small fragments of root. This ability to grow from small root fragments deep in the soil makes some thistles especially difficult to clear from pasturelands.

The thistle is a national symbol of Scotland. For this reason it was also adopted as the symbol of the Encyclopædia Britannica, which was first published in Edinburgh, Scotland, in 1768.