What are the differences and similarities in probability and non-probability sampling?

Answer 1

Probability based sampling involves picking a sample from a large collection so that individual representations within that sample are the same as of the larger population. For example, if you wanted to pick a sample of students from a university where you know women :men are in the ratio 2:1, then you would ensure that your sample also has the same proportion. In "non-probability" based sampling you would randomily select users with no respect for the preserving the proportions.