Drag the left mouse button in the graph panel. A rectangle is shown and all nodes intersecting with the rectangle are selected when the mouse is released. Selected nodes are made visible with four little black squares at the corners (see screenshot above).
The selection can be extended by keeping the shift key pressed while selecting another set of nodes.
The selection can be toggled by keeping the ctrl key pressed. All nodes in the selection selected in the rectangle are de-selected, while the ones not in the selection but intersecting with the rectangle are added to the selection.
Groups of nodes can be moved by keeping the left mouse pressed on one of the selected nodes and dragging the group to the desired position.