PBS is the acronym for phosphate buffered saline. Phosphate buffer is one of the most common buffers used in biological research. The phosphate serves as a buffer to keep the pH constant, while the saline is referencing the osmolarity. Additional ions such as Ca2+ or Mg2+ , energy sources like glucose, or chelators such as EDTA can be added based on the specific needs of the experiment.
There are many different formulations of PBS, based on the cell type and needs. The most common, from Cold Spring Harbor is shown below, providing a pH of 7.4 with an osmolarity to normal human cells.