The Bourgeoisie or the Capitalist class are the ones who own and control the wealth of a country. These control the productive forces in society (what Marx called the economic base), which basically consisted of land, factories and machines that could be used to produce goods that could then be sold for a profit.
The majority, or the masses, or what Marx called The Proletariat can only gain a living by selling their labour power to the bourgeoisie for a price