A node is the holder of an Operator and all its associated information in a TouchDesigner network. Nodes include information such as flags, inputs and outputs, the OP icon and the OP Viewer.
Nodes can be re-sized by left-clicking on their borders and dragging the desired edge in and out. 'Node' and 'Operator' can be used almost interchangeably when describing TouchDesigner networks i.e. 'dropping a node' or 'dropping an OP' are both valid descriptions of the placement of an Operator in a network.
Node is generic, operator is the specific entity that does the work of generating the data output of the node.