SBR is more formally known as Styrene Butadiene Rubber. It has an interesting chemistry, with different forms used for different purposes, but in the paving trade, it's most common use is to 'beef up' mortars and concretes. 

In our trade, it's most commonly used in its form as a thickish, gloopy white liquid, which echoes its roots as a synthetic latex.

It is called styrene butadiene copolymer latex admixture, which is SBS latex. 

It is not 'pure' SBR, but a co-polymer which has been doctored and doped and diluted to give it the properties we need in the construction industry, that is designed as an integral adhesive for cement bond coats, mortars and concrete to improve bond strength and chemical resistance.