Answer: Ganglia are mostly made up of neuronal cell bodies and dendrites, while nerves are mostly made up of the axons.
When dissecting the nervous system outside of the brain, it is common to come across large clumps of neuronal cell bodies (also called soma). These clumps are called ganglia (the singular form is ganglion).
Ganglia are a part of the peripheral nervous system, or the PNS. The PNS is divided into three branches. The somatic peripheral nervous system is responsible for communicating to and receiving signals from the surroundings. There are sensory ganglia, such as the dorsal root ganglion. On a microscopic level, the dorsal root ganglion contains the somata of the sensory, or afferent, neurons. These cells are pseudounipolar neurons because they extend their single axon in two different directions: one towards the skin for detecting tactile sensations for example, and the other towards the spinal cord. Historically, these have also been called the spinal ganglion since they are located close to the spinal cord.
Another component of the peripheral nervous system is the autonomic nervous system, or ANS. The ANS communicates with the internal environment, such as by monitoring the condition of the lungs or stomach. The nerves that communicate to and receive information from these organs often bundle their cell bodies into ganglia as well.
Some of the cranial nerves also have ganglia. Cranial nerves are the 12 pairs of nerves that exit or enter directly from the brain. Many (but not all) are responsible for communicating with the muscles or sensory components of the face, such as the muscles of the eyes or the sensory system that allows for smell. One of the most prominent cranial nerve ganglia is the trigeminal ganglia, which houses the cell bodies of the trigeminal nerve. The trigeminal nerve is important for several functions, including sensing touch information on the skin of the face and controlling the muscles of the jaw and mouth.
Be aware that there are structures in the brain (central nervous system) that are also called ganglia, namely the basal ganglia. The basal ganglia are a series of forebrain structures that are not part of the cortex, and are therefore classified as subcortical. The basal ganglia consists of circuits which connect several brain structures, including the striatum, amygdala, hippocampus, and many others. The basal ganglia is implicated in many different disorders, such as Parkinson’s disease, substance use disorder, and OCD.