Emulsifying wax (which can be either plant- or petroleum-based) is an ingredient that emulsifies the ingredients in a cream or lotion product, thus preventing them from separating in the mix. Emulsifying wax produces a much more reliable, stable emulsion than a beeswax/borax combination.
Emulsifying wax is created when the wax material (either a vegetable wax of some kind or a petroleum-based wax) is treated with a detergent (typically sodium laurel sulfate (SLS) or polysorbates) to cause it to make oil and water bind together into a smooth emulsion.
If using SLS-free ingredients is important to you, check with your supplier to make sure the wax they sell does not contain SLS. Likewise, if you prefer not to use petroleum by-products in your cosmetics, ask your supplier if the emulsifying wax they sell is petroleum based.