What is the Practical Difference between DOS & DDOS attack???

In a DOS attack, a single computer and single internet connection is used to flood the target with packets so the target server is overloaded with packets and eventually becomes unreachable.

In a DDOS attack,  several computer and several internet connections are involved in flooding the target with packets which are often distributed globally and usually called as Botnet. This kind of attack is much harder to deflect because there is no single attacker to defend.