Wikipedia offers 3 equivalent definitions of recursively enumerable languages and states closure properties for certain operations.

Two theorems which show that the class of languages accepted by Turing machines is the class of Type 0 or recursively enumerable languages.

A site which shows that the languages generated by phrase structure grammars are recursively enumerable languages. Similarly, it shows that the languages generated by context sensitive grammars are accepted by linear bounded automata.

This site discusses denumerable sets and recursive and recursively enumerable languages

This chapter shows that the family of regular languages is a proper subset of context free languages and the latter is a proper subset of recursively enumerable languages.