The CIA's World Factbook defines a head of state as an individual who represents a government in a symbolic fashion but does not oversee the daily activities of a country. A head of government, on the other hand, oversees daily executive and legislative activities of a country.
The differentiation in terms is important to recognize in governments with a parliamentary system, such as the United Kingdom, Canada and Australia. The head of state may be the monarch, whereas the head of government may be the prime minister.
Although the head of state usually does not hold any type of executive or legislative role, she holds a symbolic role. A head of state acts as a symbol of national identity and helps promote unity and pride. She also acts as the symbol of the nation for the international community, receiving and hosting foreign ambassadors. The ability to pass legislation is left in the hands of the head of government.
The head of government oversees the operation of the civil service and government agencies and appoints members of the government. In a presidential system, a single individual holds both titles and does both sets of duties. This is seen in the United States, South America and some African countries.