Nodes are the junctions in the network; with respect to river networks they represent sources, mouths, tributary junctions and the top or bottom of loops.  RivEX can extract out the various types of nodes into their respective layers.  The create pseudo node free tool can be used to simplify a networks complexity by dissolving lines that join at a pseudo node, this can in some cases significantly reduce dataset size and boost performance without loss of detail to the geometry of the lines.  The two node tools are: