Blood clots are gel-like clumps of blood. They are beneficial when they form in response to an injury or a cut, plugging the injured blood vessel, which stops bleeding.
Some blood clots form inside your veins without a good reason and don't dissolve naturally. These may require medical attention, especially if they are in your legs or are in more critical locations, such as your lungs and brain. A number of conditions can cause this type of blood clot.