What does simple, stratified, systematic, convenience, and voluntary mean in mathematics?

The terms simple, stratified, systematic, convenience and voluntary are used to refer to samples therefore, in statistics. Simple sample is a type of sample that which is most likely to be chosen. The stratified sample refers to the  grouping of samples. Systematic sample is when sample is taken with exact interval unit. Convenience sample is a sample with accessible population items. Voluntary sample is a sample made up of willing participants.