Before I got into medicinal mushrooms I had never heard of polysaccharides or beta-glucans, but these compounds are not unique to fungi, and instead are widely pervasive.
There are very different functions among polysaccharides and even beta-glucans. The aim of this article is to clear up the topic.
Polysaccharides are carbohydrates. The name means many (poly) sugars (saccharides).
But they are not like your monosaccharides like glucose and fructose or disaccharides like sucrose, common table sugar. When more and more of these and other sugar molecules are linked together the function changes.