A concussion is an injury to the brain that comes from a solid hit to the head (such as getting tackled in football) or a shockwave (such as an explosion). The knock to the noggin causes the brain to malfunction. Researchers aren't completely sure how a concussion works, but it definitely affects the brain's ability to function correctly.
The symptoms of concussions vary widely and make diagnosing a concussion difficult. Special training can help coaches and athletic trainers recognize danger signs and keep kids safe by identifying which kids need to see a doctor before returning to play.
The worst part about concussions is the damage that's done if you sustain multiple concussions over time. Research, especially from professional sports and the military, is beginning to show just how much damage a bunch of concussions can do. For instance, multiple concussions can lead to a condition very similar to Parkinson's disease.
All concussions should be evaluated by a health care provider because while a concussion is considered the least dangerous traumatic brain injury, it is still a traumatic brain injury.
Makdissi M. "Sports related concussion - management in general practice." Aust Fam Physician. 2010 Jan-Feb;39(1-2):12-7. PubMed PMID: 20369128.