What are the most common buffers used for agarose gel electrophoresis?

The most common buffers used for agarose gel electrophoresis are those that contain either Tris-acetate-EDTA (TAE) or Tris-borate-EDTA (TBE).

Acetate and borate are acids, whereas Tris is a strong base. The acid-base combination helps to keep the pH in a near-neutral range of about 8 to 8.5. The mildly basic condition protects the DNA and allows it to separate properly. 

EDTA (Ethylenediamine tetraacetic acid) plays an integral role in these buffers. It protects DNA from the cleaving activity of the DNAse enzyme. It also protects DNA by neutralizing the charge of water molecules.