A blood clot - basically just a hardened lump of blood - is formed when blood converts from a liquid to solid form by a process known as coagulation.
The medical profession, fond of hair-splitting labels, is not content to leave a blood clot as just a blood clot. So, a thrombus is a clot which forms inside of a blood vessel, but if that clot moves from its place of origin it is referred to as an embolus.
The presence of arteriosclerotic plaque lining the blood vessel walls will stimulate clot formation, with potentially fatal consequences if the clot slows down or even stops the flow of blood through the vein.