National Assembly is usually an assembly composed of the representatives of a nation. Its main characteristic is that it represents the whole population of a country and is a fixed legislative integrated part of the nation's supreme government. It may either possess all the gonvernmental powers, or it may work only within the legislative branch of the gonvernment (making and enacting the laws).
A National Convention, however, has a more extraordinary nature. A great example of one is the French National Convention that took place in 18th Century French, created to temporarily suspend the king's authority (King Louis XVI) and create a non-monarchical constitution. It also decreed that all deputies of that convention should be elected by vote (with this, being the first French aseembly elected by sulfrage).
So both of them represent a high position within a national government, related to its legislative aspect. Both have the power to create and promulgate laws, however a National Assembly does not have the power to alter the country's constitution, it works under it, instead. A National Convention, on other hand, besides having this extraordinary aspect (is not an integrated fixed part of the government), may change the constitution as the mentioned French example did.
