This is a part of a website address that you enter into a web browser (e.g. transworldafrica.co.ke or google.com) to find a web page on the internet. It is also the last part after the @ sign in an email address.
To get a domain for your organisation, or for your blog, you must register it with an authorised domain registrar like TransworldAfrica.co.ke. The domain name will point to your website, and also enable you to have custom email addresses like firstname.lastname@example.org
If I register one, is it mine for life? When you register a domain it is usually registered for an initial period of either 1 year or upto 5 years. Upon expiry of the initial registration period, you need to renew your domain with your registrar. The domain remains yours as long as it registered and active.