A typical car battery can supply between 200 and 600 amps of current for a short period of time.
This is enough to kill you or give you a serious electric shock or burn.
When changing the battery in your car you should always disconnect the cable from the negative (black) terminal first.
The reason for this is that the negative terminal is connected to the chassis of the car. This is a very large metal surface and is easily touched by accident.
If you disconnect the cable from the positive (red) terminal of the battery first then there is a chance that you might touch the positive terminal while touching the chassis (which is still connected to the negative terminal). This would complete the circuit with you in the loop and you could receive a serious electric shock.
Positive -> You -> Chassis -> Negative
You should also remove any metal rings, watches, bracelets, etc. you are wearing in case you short circuit the battery with them. The current could cause the metal to heat up and melt very quickly causing a serious burn.
When replacing the battery after charging or installing a new battery you should connect the positive terminal first and the negative terminal second.