Redirection
Redirection is the shell's mechanism for opening, closing, duplicating, and performing other manipulations on file descriptors. Redirection is either the last or second-to-last (depending on the shell and command) step of simple-command evaluation prior to running the command itself.
Links
Illustrated redirection tutorial (recommended), redirection reference, and understanding the copy descriptor on the bash-hackers wiki.
The Bash manual includes complete documentation. A quick reference is available on the BashSheet.
FAQ 55 provides a basic introduction, and the BashGuide has a very short non-technical introduction.
The portable types of redirects are specified by POSIX