A ganglion is an anatomical structure which describes two unrelated things :
The lymph node (which is what is generally being referred to when the term ganglion is used without any other qualification) is a nodule arranged along a lymph vessel in certain parts of the body. Lymph nodes filter lymph and are involved in the body's immune defences. They may be affected by cancer (lymphoma).
Nerve ganglion of a 7 day old chick. © Wikipedia CC by sa 2.5