An "itis" is an inflammation of whatever the thing is in the word that's before the "itis". So meningitis is inflammation of the meninges (mening-itis). The meninges is a three-part layer of connective tissue that surrounds the brain and descends from the skull to surround the spinal cord all the way down the spine. Inflammation of the meninges (meningitis) can be caused by a variety of infectious, autoimmune, chemical, or cancerous diseases.
The most common cause of meningitis is viral infection. A variety of viruses cause meningitis, most of them cause substantial discomfort but are not life-threatening, so-called "viral meningitis". It is most common in the summer, and patients often have exposure to children (from where they get the enterovirus or other virus causing the infection). Some viral meningitis is very dangerous, such as that caused by Herpes Simplex Virus.
Bacterial meningitis is a life-threatening emergency. A variety of bacteria cause the infection, but the most common in adults is Streptococcus pneumonia. Young adults (often college aged) and military recruits are prone to Neisseria meningitidis meningitis, which is caused by close contact of large numbers of non-immune people. A vaccine is available to prevent the latter infection. A variety of other bacteria can cause it as well. Bacterial meningitis presents with intense headache, fever, and a rigid neck--patients can't bend their neck at all. Viral meningitis tends to be milder, but still there is lots of overlap, so the only way to distinguish life threatening from more benign infection is by doing a spinal tap (AKA lumbar puncture in medical lingo).
Fungal meningitis is seen in certain geographical areas in the US (southwestern US sees lots of cocci meningitis), or in patients with compromised immune systems. TB can also cause chronic meningitis.
A variety of non-infectious diseases, such as systemic lupus erythematosis, systemic vasculitis of a variety of types, medication reactions (NSAIDs, sulfa drugs, etc.), chemical reactions to blood in the cerebralspinal fluid (such as occurs after a ruptured aneurysm), and leukemias, lymphomas, and certain carcinomas (such as breast) can also cause meningitis. A spinal tap is necessary to make the diagnosis.
Treatments vary by underlying disease.