Domains are registered with your domain provider register. There may be reasons due to which you may need to change your register.
There are 2 aspects of transfer : –
Registrant Domain Name Transfer: It involves transferring all the domain name rights to a third party. Only domain name registrant or the administrative contact can do this.
Registrar Domain Name Transfer: It involves the current domain name registrant wanting to change his domain registration company. This request should go through the new registrar who will contact the current one for authentication details and then take over the domain under his registry.