to put some text next to the b node look at this example:
digraph g{
  a -> b
  a [label="hello",shape=ellipse,fillcolor="burlywood",style="filled"];
  b [label="graphviz",shape=box,fillcolor="palegreen",style="filled,rounded"];

b->iuc [style=dotted, arrowhead=odot, arrowsize=1] ;
iuc [shape=plaintext,label="This is a test of a long\n comment and how dot is showing\n it"];
{rank=same; b;iuc}
}

There is no direct support for this, though there are various ways to approximate it. One way is to attach an xlabel attribute to the node (this requires a version of Graphviz later than August, 2011). This will position the label exterior to the node, avoiding overlap with other nodes and labels. Here, you don't really have control over where the node is placed. Alternatively, you can use HTML-like labels to represent your node with an apparently external label and the node proper. You also might be able to use an edge headlabel or taillabel. For example, in a dot layout, with edges pointing down, attaching a taillabel to an edge leaving a node will appear as a label beneath the node.