A muscle strain, also called a pulled muscle, happens when a muscle is stretched too far and part of it tears. According to MedlinePlus from the U.S. National Library of Medicine, the main symptoms are pain and difficulty moving the injured muscle, discolored or bruised skin, and swelling.
Strains are commonly caused by too much physical activity, not warming up properly, poor flexibility, or falls. MedlinePlus advises calling 911 if you cannot move the muscle or the injury is bleeding, and contacting a health care provider if the pain does not go away after several weeks.
Source: MedlinePlus Medical Encyclopedia: Strains
Image: Bruising from a torn hamstring, photo by Jabarnes at English Wikipedia, public domain, via Wikimedia Commons.