Generically speaking it is a long fiber used for making paper products out of a variety of materials including wood, cotton, and other plant materials. What's special about alpha-cellulose is that it is devoid of any of the acid-producing structures like lignin allowing for high quality archival papers to withstand the test of time and remain acid-free.

Technically speaking, it is the strongest, most durable form of polysaccharide chains derived from plant matter. What matters is that it makes great, archival paper.