A domain name is a name that you reserve the right to use inconjunction with web servervices for a fixed period of time. Miminum registration is one year, maximum ia usually 10 years.
You register your domain with a "registrar" which charges a registration fee to keep track of your registration record. which comtains your registration date, expiration, name, address, phone,
email and where your domain is pointed and they will offer a wide range of other domain related serverices.
** Domain names are all lower case. If you see mixed case domain in links or on billboeards, that's just for readability.
** Domain names are comprised of one string of letters, numbers and may include a dash or hyphen.
Once your connect your domain to your site or host, you may also "map" or add sub-domains, like many.yourdomain.com