It all depends on where it forms.
When a clot forms, it can either partially or totally block the blood flow in that vein. Symptoms of a dvt can include:
swelling of the leg
warmth and redness of the leg
pain that is noticeable, or worse when standing or walking
these are not always a sign of a dvt, but anyone who experiences them should contact a doctor immediately.
Symptoms of pulmonary embolism
these include shortness of breath; chest pain which may be worsened by deep breaths; and coughing up phlegm, possibly messed with blood.
Anyone with these symptoms should seek emergency medical treatment.