As a co-owner of a domain registration company i posses the knowledge about frequently asked questions about domain registrations. One of those questions is about "WHOIS".
Lets start with a quote from wikipedia.org:
"WHOIS (pronounced as the phrase who is) is a query and response protocol that is widely used for querying databases that store the registered users or assignees of an Internet resource, such as a domain name, an IP address block, or an autonomous system, but is also used for a wider range of other information. The protocol stores and delivers database content in a human-readable format."
Why is knowing about WHOIS important?
Everything is done on the internet these days. This includes shopping, chatting but even important stuff for which you had to travel to a government building a couple years ago. Because of this we can imagine the number of people who register a domain name in order to build a website on it, forward it to another website or amongst the many other reasons try to sell it.
What most people however don't know is that when you register a domain name, your contact information will be shown in the WHOIS database. This way people (including annoying spammers) can find your contact information. Not all websites who provide you with this information will always show you the same information. In the most cases, when you don't have any special WHOIS protection, people who do a WHOIS check on your domain name will be able to find the following information:
Domain Information
Domain name: The domain name which is being checked.
Registrar: The company where you have registered your domain name with, (for example: Go Daddy).
Registration date: The date when this domain has been registered.
Expiration date: The date when this domain will expire in case the customer decides to stop the contract with the registrar.
Updated date: This shows the date when the WHOIS for this domain has changed for the last time.
Status: The status of this domain name
Name servers: The name servers of this domain.
Registrant Contact (The owner of the domain)
Name: The name of the person who owns this domain name.
Organization: In case the domain name is registered under an organization name, this will show which organization it is.
Street
City
Postal Code
Country
Phone
Fax
E-mail
Administrative Contact
In most cases shows exactly the same as Registrant Contact
Technical Contact
Exactly the same as Registrant Contact but with the information of either the Registrar or a technical contact chosen by the Registrant.
Why am informing you about this?
As someone who talks to his customers alot, which are all owners of domain names, i have noticed that many people do not realize how much of their information is out there on the internet without them knowing about it. Our company does inform the customers about it, but registrars are not obligated to do this.
What can you do as a domain owner?
All registrars should be able to hide your whois from the public. If you want to see an example, you could try to do a whois check on steemit.com by using whois.com. You will see that none of registrant/administrative contact are being shown except for the organization name. Because of this, nobody can go and spam 
In case you own a domain name and you don't want your information to be shown on the internet for everyone to see, make sure you contact your registrar and ask them for whois protection.
WHOIS can be good and also be bad
It's obvious why it can be bad, people can find your contact details which can result in huge amounts of spam or in some cases troubles. The good part about Whois is that selling a domain name gets alot easier. People can contact you if they are interested in your domain name. On top of that, which can be more important is that it allows people who would like to work with your or start a partnership can find your contact information. To be honest, the above is not needed at all if you just make a 1 page website with your contact information on said domain name(s). This could be very expensive if we are talking about more than just a couple names ofcourse.
Thank you for reading
Thanks everyone who have read this, if i have helped you with this i would love to see a comment in the comment section below. I'm thinking about sharing more information about registering domain names and everything about and around it, so make sure you follow me if that's interesting for you!