Iterators
SQL Server uses an extensible architecture for query optimization and execution, using iterators as the basic building blocks.
Iterators are probably most familiar in their graphical showplan representation, where each icon represents a single iterator. They also show up in XML query plan output as RelOp
nodes:
Each iterator performs a single simple function, such as applying a filtering condition, or performing an aggregation. It can represent a logical operation, a physical operation, or (most often) both.