Venn diagrams are pictorial representations of sets that can be used to compare and contrast characteristics of different groups. When teaching children, manipulatives are often used before drawing the diagrams; attribute blocks (which come in different sizes, shapes, colors, and thicknesses) can be sorted and placed in overlapping loops of string. Venn diagrams look (and work) something like this:

Set A Set B
_____________ __________________
/ Letters which \ / Letters containing \
/ contain curves \ / straight lines \
/ \/ \
/ S /\ T \
| C / \ |
| | B | H |
| | P | E |
\ O \Q / V /
\ \/ L /
\ /\ K /
\ ______________ / \ ___________________ /

(Except, you know, curved, and sometimes with more than two circles)

As you can tell from this marvelous ASCII demonstration, the intersection of ** Set A ** {S, C, O, B, P, Q} and **Set B** {T, B, E, P, H, Q, V, L, K} is the subset that contains letters with characteristics of *both* sets, namely B, P, and Q.

Venn diagrams are traditionally used in mathematics (see set theory), but these and other graphic organizers have found their way into elementary school science, social studies, and reading classrooms as a way to organize information (similar to brainstorming) before writing.

Venn diagrams for classroom use:
http://www.graphic.org/venexp.html
More techical explaination and cool graphics: http://sue.csc.uvic.ca/~cos/venn/VennWhatEJC.html