Brain stem gliomas are primary tumors occurring in the brain stem, the deep central/posterior part of the brain that connects the rest of the brain to the top of the spinal cord. The brain stem consists of the medulla, pons, and midbrain. This critical area of the brain controls many of the vital functions of life such as heart rate and breathing, as well as nerves that control vision, hearing, speech, swallowing, and movement of both arms and legs. The term “brain stem glioma” most commonly refers to a malignant, diffuse (spreading) tumor that involves the entire pons (also called a diffuse pontine glioma)
Source : Brain Tumor Society (archived)

These cancerous tumours account for around 10 - 15% of paediatric brain tumours and around 1% in adults.

The cause is unknown.

Further info Wikipedia