What Is The Difference Between De Facto And De Jure Sovereignty?

This is a feature of international law.  Sovereignty is the right to exclusive authority, usually political and usually by the state as opposed to an individual.
De jure sovereignty is the legal right to exert this power.  The phrase is translated as by the jury, meaning based on law or commonly known as in principle.
De facto sovereignty is how sovereignty works in practice.  This phrase is translated as 'in fact'.  In other words,  the reality of sovereignty.
in day to day terms, de facto and de jure sovereignty are united.  The state usually finds that it does not have a problem exercising its power, so it works in practice.
However, if the majority of the people suddenly oppose the state then it may have 'de jure' sovereignty, but it will not have 'de facto' sovereignty since it will not be able to enforce laws, power etc effectively.
This can also be an issue where there is a question over the legality of  government or where there is political instability.  Currently there are fears in Iraq that the 'de facto' sovereignty of the Government will be undermined by the presence of foreign troops which make Governance difficult in practice.