Documentation can be found for various (tcllib) data structures. Provided structures, so far:
Often what people want from "struct" is a C-like datatype. This is much different from the tcllib data structures described above. Various people have coded Tcl extensions for this kind of "struct"; Iain B. Findleton's Containers appears to be a very nice data structure extension that includes one.
Many people have written small procs that implement a C like structure facility in Tcl. Check google nearly monthly for discussions. Here's a thread [L1 ] that covers one nice implementation.
The first exchange in that thread is also here at C Struct is Tcl!