category with duals (list of them)
dualizable object (what they have)
n-category = (n,n)-category
n-groupoid = (n,0)-category
String diagrams are a graphical calculus for expressing operations in a monoidal category. The idea is roughly to think of objects in a monoidal category as “strings” and a morphism from one tensor product to another as a node which the source strings enter and the target strings exit. Further structure on the monoidal category is encoded in geometrical properties on these strings. For instance
putting strings next to each other denotes the monoidal product, and having no string at all denotes the unit;
braiding strings over each other corresponds to – yes, the braiding (if any);
bending strings around corresponds to dualities on dualizable objects (if any).
Many operations in monoidal categories that look rather unenlightening in symbols become very obvious in string diagram calculus, such as the trace: an output wire gets bent around and connects to an input.
There are many additional structures on monoidal categories, or similar structures, which can usually be represented by encode further geometric properties. For instance:
in monoidal categories which are ribbon categories the strings from above behave as if they have a small transversal extension which makes them behave as ribbons. Accordingly, there is a twist operation in the axioms of a ribbon category and graphically it corresponds to twisting the ribbons by 180 degrees.
in a traced monoidal category?, the trace can be represented by bending an output string around to connect to an input, even though if the objects are not dualizable the individual “bends” do not represent anything.
in monoidal categories which are spherical all strings behave as if drawn on a sphere.
there is also a string diagram calculus for bicategories, which extends that for monoidal categories regarded as one-object bicategories. Thus, the strings now represent 1-cells and the nodes 2-cells, leaving the two-dimensional planar regions cut out by the strings to represent the 0-cells. This makes it manifest that in general, string diagram notation is Poincaré dual to the globular notation: where one uses -dimensional symbolds the other uses -dimensional symbols.
A string diagram calculus for monoidal fibrations can be obtained as a generalization of C.S. Peirce’s “existential graphs.” The ideas are essentially contained in this paper by Brady and Trimble, and was discussed here.
String diagrams for closed monoidal categories are similar to those for autonomous categories, but a bit subtler, involving “boxes” to separate parts of the diagram. They were used informally here, but can also be done in essentially the same way as the proof net?s used in intuitionistic linear logic; see this paper. Proof nets for classical linear logic similarly give string diagrams for *-autonomous categories.
See the article by Selinger below for more examples.
John Baez and Mike Stay, Physics, Topology, Logic and Computation: A Rosetta Stone, arXiv
The Catsters (Simon Willerton), String diagrams (YouTube)
Probably David Yetter was the first (at least in public) to write them with “coupons” (a term used by Nicolai Reshitikhin and Turaev a few months later) to represent maps which are not inherent in the (braided or symmetric compact closed) monoidal structure. See also these:
David Yetter, Framed tangles and a theorem of Deligne on braided deformations of tannakian categories In M. Gerstenhaber and Jim Stasheff (eds.) Deformation Theory and Quantum Groups with Applications to Mathematical Physics, Contemporary Mathematics 134, pages 325–349. Americal Mathematical Society, 1992.
For more on the history of the notion see the bibliography in (Selinger 09).
Ross Street, Low dimensional topology and higher-order categories – talks about surface diagrams and includes some pictures (PS version only).
Ross Street, “Categorical structures” – discusses string diagrams for bicategories.
This article discusses the generalization of string diagrams to one dimension higher: