From OpenCog

The BondNode is a type of LexicalNode used to indicate the mating of some kinds of Connectors. See Connectors and Sections for details.

A BondNode corresponds to the concept of a Link Grammar link type. In the current terminology, Link Grammar could be called "Bond Grammar", but only to avoid confusion with the Atomese Link.

There is not much difference between LexicalNodes and BondNodes, except that the size of the lexis for BondNodes tends to be quite small -- nor more than a few hundred, typically. i.e. a "closed class" or "closed lexis", of a fixed size, whereas LexcialNodes have an "open" or unbounded size.

The TypeNode is an example of a BondNode.