Help using Cast (Text) Version
If you can point me towards a doc that explains this in simple terms I would be obliged. Don't expect you to have to provide the answer.
Any of the introductory texts should explain the various forms of indexing and the use of the apply family of functions. They are both central to effective R programming.
See also the plyr package, http://had.co.nz/plyr, which tries to provide a more uniform interface to the apply family of functions. I've also tried to document everything in one place, so hopefully it's a bit easier to learn and to see how all the different functions fit together. Hadley