Domain Names are purchased via a registrar and when registered they are purchased for a length of time in years. Each registrar has its own TAG.

123REG = 123 REG
GODADDY = GoDaddy
CHAMELEON = Chameleon
EXTEND = Heart Internet - our white label tag for clients
HEART = Heart Internet

When the expiry date comes up a domain needs to be renewed again for a length of time in years.

For .co.uk domain names the best solution for checking is:-

https://www.nominet.uk/whois/

This is the most accurate as Nominet as this is the official registry for all .UK domain names.
The WHOIS lookup tool searches our directory and returns information about the domain name searched for.
The WHOIS lookup tool will return domain information for .uk, .co.uk, .org.uk, .me.uk, .net.uk, .sch.uk, .plc.uk, .ltd.uk, .cymru and .wales

For .com domain names our any other weird ones the best solution for checking is:-

http://whois.domaintools.com/

Creation Date: 201?-??-?? - This is the date the domain name was first registered or re-registered after it dropped.

Registry Expiry Date: 201?-??-?? - This is the important date - this is when the domain name will expire. If it is not renewed it will drop after 90 days and then anyone can register it.

Updated Date: 201?-??-?? - This is the date that the domain name record was last touched.