Most countries have a Head of State, the ceremonial functionary - opening Parliament, shaking hands with foreigners etc - and a Head of Government who governs, or likes to think he/she does.

Examples:
In the UK, the HoS is the monarch (whether King or Queen), the HoG is the Prime Minister, currently Mr Blair.

In Germany, the HoS is the President - currently Dr Rau, the HoG is the Chancellor, currently Mr Schroeder.

For countries such as the US and France, the situation is muddied because the HoS is also the HoG. (OK, I know the French have a prime minister as well as a president, but observing from the outside, it seems Mr Chirac pulls all the strings .)

Confusing, isn't it.