The spleen mainly "recycles" the old erythrocytes, or red blood cells in your body. Red blood cells function properly for approximately 120 days after they are formed in the bone marrow. Towards the end of this lifespan, they become damaged and inefficient. This is when they make their way to the spleen, where they are broken down and the matter they are composed of is used in the creation of new erythrocytes. This process can also occur in the liver.
The spleen in human, it acts primarily as a blood filter. The spleen also participates in the creation of blood cells as well as fight infection. In humans, it is located in the left upper quadrant of the abdomen. Read more on http://en.wikipedia.org/wiki/Spleen.