This distinction is generally made in times of revolution or war when the legally constituted government is overthrown and a new authority assumes power by force, having no legal claim to power. The former would be known as de-jure and the latter as de-facto sovereign.
Thus de-jure sovereign is one who has a legal claim to sovereignty but does not possess it in fact while de-facto sovereign is one who has no legal claim to sovereignty but possesses it in fact and exercises necessary force to make and enforce its laws.
After the lapse of some time, when the de-facto sovereign shows sign of permanence and continuity and is able to acquire popular consent, either by democratic methods or by force, it will become de-jure sovereign as well and will be so recognized by other states.
The difference is thus temporary. Either the de-jure sovereign regains its authority or the de- facto acquires legal sanction and becomes de-jure sovereign.
To take an example, the Communist Government in China remained de-facto sovereign whereas the National Government under Chiang Kai-sheik was de-jure government. Since the Communist Government of China has now been recognized by the international community, it has now assumed the position of a de-jure sovereign as well.