http://www.cancerbacup.org.uk/Treatments/Stemcellbonemarrowtransplants/Generalinformation/Bonemarrowandstemcells
from the link above:
Bone marrow and stem cells
Bone marrow is a spongy material that is found inside the bones (particularly the pelvic bones). Like a factory, bone marrow produces the cells which develop into the three different types of blood cells:
Red blood cells which carry oxygen to all cells in the body. White blood cells which are essential for fighting infection. Platelets which help the blood to clot and prevent bleeding. Stem cells are blood cells at the earliest stage of development in the bone marrow. Within the bone marrow stem cells develop into the different blood cells described above. When the cells are fully mature they are released into the bloodstream.
Normally, most of the stem cells in the body are in the bone marrow and there are only very small numbers in the bloodstream. However, it is possible to stimulate the stem cells to move into the bloodstream, by using low doses of certain chemotherapy drugs or injections of proteins known as growth factors. Stem cells can be collected from the bone marrow or from the bloodstream.
Throughout this section, we use the term 'bone marrow' to mean stem cells which are obtained from the bone marrow, and the term 'stem cell' to mean stem cells obtained from the blood. These are the terms doctors often use. In fact, in both cases it is blood stem cells that are being collected.
