A B-Tree of order m is a search tree, where the data 
  (or pointers to it) is stored at the leaves, such that:

  1) the root is either a leaf (as well--i.e. the tree is 
     a single node) or has between 2 and m children

  2) all other (non-root) nodes have at least m/2 children,
     but no more than m children

  3) all leaves are at the same depth
Since each interior node has as many as m pointers to its children, they must also keep "search" data to point the search in the right direction.

So, an interior node with n children, will have n pointers (p1 to pn) to those children; it will also have n-1 keys (k1 to kn-1), where ki is the smallest key stored in the tree pointed to by pi+1.

Note that some entries in a node can be null--its just important that the key and its matching pointer are both null.