Students' Unions are charities, run independently from the university whose students they work for.
Their role is to represent the best interests of students of the university both within the university itself, and on national issues.
Students' Unions are run by elected student officers, both part-time and full-time. These officers lead the work of the Union, using student input to direct their efforts.
Students' Unions also have full-time staff teams, who help make sure the Union is running effectively by carrying out a lot of the behind the scenes admin.