What is a lymphocyte cd20? Is it a type of cancer marker?

Yes. Most of your normal b-cells bear this marker. It's typically used in subclassifying malignant lymphomas that have already been diagnosed, in combination with a huge array of other markers, though it can be used to "light up" the distribution of b-cells in non-cancerous tissue as well. It's the target for Rituximab and some other new rx's.
It is a major. B-lymphocyte marker, carried by benign cells as well as malgnant cells with b-cell lineage (as opposed to t-cell lineage). The ab Rituximab has been generated against it and is a key component in treating b-cell lymphomas.