Positive selection involves targeting the desired cell population with an antibody specific to a cell surface marker (CD4, CD8, etc.). The targeted cells are then retained for downstream analysis.
Negative selection is when several cell types are removed, leaving the cell type of interest untouched. Similar to positive selection methods, cells are labeled with antibodies that target specific cell surface markers or populations. However, in the case of negative selection, the unwanted cells are the ones labelled and then subsequently removed.