Why do I always hear the term "concussion" but I never really know what happens when you get one. Anyone know?

Bruised brain. A concussion is, in a sense, a bruise on your brain. When you are struck or your head suddenly jerked, your brain is "rattled around" inside your skull. This damages blood vessels, causing the damage that results from a concussion. Here is a good summary of concussion. http://www.webmd.com/brain/tc/traumatic-brain-injury-concussion-overview.
Loss of consciousness. Concussion refers to an immediate and transient loss of consciousness accompanied by a brief period of amnesia after a blow to the head. The extent of the amnesia roughly correlates with the duration of loc and the severity of head injury. There is both anterograde amnesia (can't retain new info) & retrograde amnesia, usually from just prior to the injury. Both improve.
Concussion. You've probably never had one - which is good! The effects of concussions can vary and be unpredictable. It depends on the severity of the injury. The symptoms can include headache, dizziness, vision change (blurred vision/ double vision), nausea/vomiting. If your symptoms or headache are severe, or if you have a loss of consciousness, you should get evaluated immediately.
More symptoms. More and more is being learned about concussions. Obvious symptoms may include fatigue, nausea, blurred vision, pain upon light exposure.And headaches. Subtle symptoms include " muddy thinking", slow reflexive response, poor school work etc.Final resolution may take months. There are validated tests and specialists for proper evaluation and they should be used.