What is the difference between a .com, .net and .org in a domain name?

Practically speaking, there is really no difference these days. Search engines don’t discriminate between a .COM address and a .NET address. The only thing you might consider is that people tend to type in .COM automatically since it was the first publicly known domain extension. So when registering a domain name, I would go for the .COM first and if it was taken, I would then try for any of the others. (.net, .org, .tv, etc. …)
You probably guessed; a .COM address is not the same domain name of the same name with a different extension. So for example:
www.killersites.com is not the same place as www.killersites.net
As such, each of the addresses can be registered separately.
- See more at: http://www.how-to-build-websites.com/basic-concepts/part2.php#sthash.WRwxyx9l.dpuf

 Practically speaking, there is really no difference these days. Search engines don’t discriminate between a .COM address and a .NET address. The only thing you might consider is that people tend to type in .COM automatically since it was the first publicly known domain extension. So when registering a domain name, I would go for the .COM first and if it was taken, I would then try for any of the others. (.net, .org, .tv, etc. …)

You probably guessed; a .COM address is not the same domain name of the same name with a different extension. So for example:

www.stardesign.com is not the same place as www.stardesign.net

As such, each of the addresses can be registered separately.

.com is what most people choose because it is the standard for business

Most non profits use .org

.net is derived from network and originally applied to organizations involved with networking technology

If I buy a website name, I buy the .org, .net, and sometimes .biz and direct them all to the .com

As to website addresses being taken, try to be creative. Think of names that utilize the benefits of your product or service. Try not to misspell anything though...people will have an awful time finding you.

One other thing: spam filters. If you go with .net or .biz, you will sometimes be blocked when sending email and when posting your web address in an email...something to consider.