These labels are often used to describe certain types of theories adopted by historians, but what do they actually mean?
Voluntarist: These approaches stress the importance of individual agency. The cause of an event is the actions taken by the participants themselves. E.g. the Russian Revolution 1917 was caused by the actions of Lenin and the Bolshevik Party
Structuralist: These approaches stress the importance of the larger structure and environment behind the individuals. The cause of an event is the conditions of the pre-existing structure. E.g. the Russian Revolution 1917 was caused by the weakened state apparatus following the Russian army's disastrous performance in WW1, which in turn allowed the Bolsheviks to seize control.
Post-structuralist: These approaches stress the importance of abstract ideas and beliefs (rather than concrete structures) and how these have been used to shape history. The cause of an event is the ideology behind it. E.g. the Russian Revolution 1917 was caused by the rise of the ability of Bolshevik Communism to connect with the Russian people over the pre-existing Tsarist absolutist ideology.