Equisetopsida is a class of vascular plants that includes the modern horsetails and their ancient relatives. These plants are characterized by their jointed stems, whorled leaves, and a unique reproductive structure called a strobilus, which produces spores instead of seeds. Horsetails are often found in wet or marshy environments and can grow in various habitats. They have a long evolutionary history, dating back to the Carboniferous period, and are considered living fossils due to their resemblance to ancient species.