Copyright can be transferred by assignment. This is the transfer of full copyright to a third party, e.g. to a publisher. In other words, copyright is passed to the publisher and the publisher then owns it and is able to exercise any exclusive rights that the copyright law gives to authors and creators.
It is not advisable for an author to sign over all their rights to a publisher. Rather give the publisher a 'non-exclusive' licence and retain certain rights that will allow you to make your works available for teaching, or on an Open Access institutional repository, etc. See: SPARC Author's Rights and Author's Addendum - link below.
Assignment must be in writing, and to be valid, it must be signed by the person assigning the copyright, i.e. the author.
A partial assignment is like the exclusive licence (see this page under 'Exclusive Licences'. The difference is that a partial assignment gives the holder copyright in a work and allows the holder (subject to the terms of the partial assignment) to grant licences and proceed directly against infringers for breach of copyright.
Copyright can be transferred by testamentary disposition, i.e. copyright passes to an heir by inheritance, as an asset in the author’s Estate. Someone who inherits an unpublished work usually inherits copyright in that work.
Copyright can also be transferred by operation of law. This can happen, for example, when the copyright holder is insolvent and the Estate is taken over and distributed among the copyright holder’s creditors.