Group of diseases. This is fairly common, caused by antibodies against, or antigen-antibody complexes adhering to, your sticky platelets. Mild or severe, the tendency to bruise and bleed prompts confirmatory lab testing. It can appear suddenly, often after a viral illness and often disappears on its own. Or it can stay on / be discovered as an chronic illness of variable severity, best managed by a hematologist.
ITP is just what... It sounds like. It is characterized by bleeding, bruising (purpura) and petechiae that develop due to a low PLT count (thrombocytopenia), and where the trigger for thrombocytopenia is not known (idiopathic). We do know that in itp, the pt makes antibodies to auto-antigens found on the PLT surface (often pla-1) which mark them for destruction. This makes PLT count low. It can be acute or chronic.