A concussion is a type of traumatic brain injury—or TBI—caused by a bump, blow, or jolt to the head or by a hit to the body that causes the head and brain to move rapidly back and forth. This sudden movement can cause the brain to bounce around, twist, or shake in the skull, creating chemical changes in the brain and sometimes stretching and damaging brain cells.
You do not have to pass out or lose consciouness to have a concussion. Some people will have obvious outward symptoms of a concussion such as:
Appearing dazed or stunned
Confusion
Forgetfulness
Clumsy movement
Slow speach
Loss of consciousness
Mood, behavior or personality changes
Cannot recall events prior to hit or fall
Cannot recall events immediately after hit or fall
Some people may feel:
Headache or "pressure" in the head
Nausea or vomiting
Dizziness or balance issues
Double or blurry vision
Sensitivity to light or noise
Feeling sluggish, hazy, foggy, or groggy
Concentration or memory problems
Confusion
Just not feeling "right" or feeling "down"
Outward signs and symptoms often show up soon or immediately after injury but this is not always the case, some symptoms may not show up for hours or days.
Further reading: CDC - Signs and Symptoms of Concussion