What is the difference between persist() and merge() in JPA and Hibernate?

JPA specification contains a very precise description of semantics of these operations, better than in javadoc:

The semantics of the persist
operation, applied to an entity X are
as follows:

• If X is a new entity, it
  becomes managed. The entity X will be
  entered into the database at or before
  transaction commit or as a result of
  the flush operation.

• If X is a
  preexisting managed entity, it is
  ignored by the persist operation.
  However, the persist operation is
  cascaded to entities referenced by X,
  if the relationships from X to these
  other entities are annotated with the
  cascade=PERSIST or cascade=ALL
  annotation element value or specified
  with the equivalent XML descriptor
  element.

• If X is a removed entity,
  it becomes managed.

• If X is a
  detached object, the
  EntityExistsException may be thrown
  when the persist operation is invoked,
  or the EntityExistsException or
  another PersistenceException may be
  thrown at flush or commit time.

• For
  all entities Y referenced by a
  relationship from X, if the
  relationship to Y has been annotated
  with the cascade element value
  cascade=PERSIST or cascade=ALL, the
  persist operation is applied to Y.

The semantics of the merge operation
applied to an entity X are as follows:

• If X is a detached entity, the state
  of X is copied onto a pre-existing
  managed entity instance X’ of the same
  identity or a new managed copy X’ of X
  is created.

• If X is a new entity
  instance, a new managed entity
  instance X’ is created and the state
  of X is copied into the new managed
  entity instance X’.

• If X is a
  removed entity instance, an
  IllegalArgumentException will be
  thrown by the merge operation (or the
  transaction commit will fail).

• If X
  is a managed entity, it is ignored by
  the merge operation, however, the
  merge operation is cascaded to
  entities referenced by relationships
  from X if these relationships have
  been annotated with the cascade
  element value cascade=MERGE or
  cascade=ALL annotation.

• For all
  entities Y referenced by relationships
  from X having the cascade element
  value cascade=MERGE or cascade=ALL, Y
  is merged recursively as Y’. For all
  such Y referenced by X, X’ is set to
  reference Y’. (Note that if X is
  managed then X is the same object as
  X’.)

• If X is an entity merged to X’,
  with a reference to another entity Y,
  where cascade=MERGE or cascade=ALL is
  not specified, then navigation of the
  same association from X’ yields a
  reference to a managed object Y’ with
  the same persistent identity as Y.