Sets of sets, which we call collections of sets.

Comparison of the properties of some collections of sets, including topologies, and the collection of closed sets in a topological space.

Definition, properties and examples of sigma-algebras.

Note: at time 21:24 I referred to (S1) when I should have referred to (S2).
I corrected this around time 22:31 when discussing infinite intersections.

For more information and links to further resources, see my blog page
https://explainingmaths.wordpress.com/measure-theory/