It manipulates entire relations through a semantically unambiguous set of operations (Ed Codd).
The relational algebra's sets are the relation's extension (tuples, rows) and the operators have either a set theoretic origin or relation oriented favour.
A relational algebraic expression has as inputs one (unary), or two (binary), relations and sometimes a selection condition.
While an expression's output is a single relation.
The algebra's operators work on all of the relation's tuples;
The algebra has a procedural computational model;
A selection condition has to be evaluated against each tuple independently;
The output of an algebraic operation is an acceptable input to a consequent algebraic expression – this is called expression composition.