What is a prime minister?

Ask the question

"Who is the Prime Minister of Canada?"

"Who is the Prime Minister of India?"

using the name of the country you are looking for.

If you mean the Prime Minister of the United Kingdom - it's David Cameron.

If you mean who is the current Prime Minister of Canada... that would be the Right Honourable Stephen Harper.

However, there are several countries around the world how have a head of government called a Prime Minister (ex: United Kingdom). There are more still that have a head of state by the same name (ex: Russia).