A random experiment is an experiment where all the possible outcomes are known, but we do not know which one will be observed.
We can list all possible outcomes into a set. Each outcome will be an element in the set. This set is known as the sample space, denoted by the Greek letter \varepsilon (pronounced as ‘epsilon’).
An event is a subset of the sample space, usually denoted by a capital letter. It usually represents a particular, smaller set of possible outcomes that has a common characteristic.
It is nice to have visual representations. Thus, we use a Venn Diagram to do so (see the example below). Each set is represented by a shape (typically, the sample space would be a rectangle, and each event is a circle).