When you think of B-trees, think “clustered index”. All indexes in SQL Server on disk-based tables are structured as B-trees. B-trees are a special case of balanced trees. A balanced tree is a tree where no leaf is much farther away from the root than any other leaf.

A clustered index is structured as a B-tree. A clustered index maintains the entire table’s data in its leaf level. The leaves are the nodes farthest from the root. The clustered index is not a copy of the data; it is the data.