Short answer
Whois is the public record of who registered a domain and through which company. A lookup shows the registrar, the registration, update and expiry dates, the status codes and the name servers, and sometimes the owner. Registries now publish this through a newer standard called RDAP, which is what modern lookup tools read.
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Whois and RDAP
Whois is the older system, a plain text service from the early internet. Its replacement is RDAP, which returns structured data over the web and handles privacy and different countries better.
ICANN, which oversees domain names, has moved registration data for global domains to RDAP. A modern lookup tool, including an ICANN lookup, is reading RDAP even when it is still called whois.
The fields, one by one
Registrar: the company the domain was bought through. It is the one to contact to renew, transfer or change the domain.
Registered, updated, expires: when the domain was first registered, when its record last changed, and when the current registration ends.
Name servers: the servers that say where the domain points. They usually show which DNS provider is in charge.
Status codes: what is allowed on the domain. The next guide explains them one by one.
DNSSEC: whether the domain's DNS answers are digitally signed to stop tampering. Many domains are not signed, and that is common.
Registrant and contacts: the owner and administrative contacts, which are usually hidden.
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Registry, registrar and registrant
Three similar words, three roles. The registry runs a whole ending such as .com or .in. The registrar sells domains to the public. The registrant is the person or company that registered the domain.
A whois lookup asks the registry, and the answer includes the registrar. The registrant is the part most often hidden.
Why the record looks different on different tools
Some tools read the registry, some read the registrar, and some show cached copies. Dates can differ by hours, and fields hidden by one are sometimes shown by another. For the authoritative answer, read the registry's own record, which is what our lookup does.
What to check on your own domain
Look at the expiry date and confirm auto-renew is on and the card is valid. Look at the status codes and check the transfer lock is on. Look at the name servers and confirm they are the ones you expect.
On one domain we run ourselves, the transfer lock turned out to be off. It takes a minute to switch on, and it is the sort of thing nobody checks until it matters.