Red blood cells are round in shape, and they supply oxygenated blood throughout the body.
Sickle Cell Disease/Anaemia has red blood cells in the shape of a sickle.
They are deformed red blood cells with decreased oxygen. Sickling of the red blood cells causes thickening of the blood, and decreases oxygen flow. Small blood vessels in different organs, bones and joints become occluded putting the individual in a painful crisis.
See further information at http://nhlbi.nih.gov/health-topics/sickle-cell-disease.