A DDOS is not that serious, all it is bassically is refreshing the target site's url or the persons ip address. If your doing it solo you'll only be able to maybe slow the site down for a short period of time and possibly deny a few users access to the site due to flooding the site url or ip rapidly using a third party program. On the other hand if you have a good number of people maybe 5 to 10 , or if your looking to do serious damage 50 + doing this all at the same time from different ip addresses, and using different ping rates every second it's possible to crash a site or tragets ip for a decent ammount of time...or untill they call their ISP and change their ip, even then if it's a site changing the ip will only work if the person or people decide to not flood the site url or ip again.
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Of course, if lead by an advanced person in ddos and cyber security/other it is also possible to do damage to the site itself, "which can be usually easily removed manually"
Yes it is illegal, but for the most part no permanent damage is usually done.