Black tea is allowed to fully oxidize during processing which gives the tea a dark color and richer flavour. Green tea is lightly oxidized so the leaves keep their original green color and the liquor is much lighter in flavour. 

Oolong tea is partially oxidised and so falls between green and black tea. The oxidation level varies depending on the production process and so some oolongs are more fresh and light and others are lean towards the deeper flavours of black tea.

Oolong tea leaves are traditionally rolled, twisted or curled into tight balls or longer strands. The way the leaf is prepared and the environmental growth conditions affect the flavour and so a vast and complex array of flavours  are available from oolong teas. 

All tea contains caffeine. A general rule is that the darker the tea, the more caffeine there is. So a greener oolong will have more caffeine than green tea, and less caffeine than a darker oolong, which in turn has less caffeine than a black tea.