"Tags" are a listing of code objects in a group of files, together with their precise location, often used by text editors to quickly jump around in a program. ctags (for C) was the first tag-generation program.
There are currently a number of different ways to generate tags with Haskell.
This page should be used to collect information on tag-generation for Haskell, including information on how to use tags with common editors and what benefits they can give you.
Haskell tag generators
Chris Ryder and Simon Thompson give a tag generator's source in a paper
echo ":ctags" | ghci -v0 Main.hs
echo ":etags" | ghci -v0 Main.hs
utils/hasktags from GHC.
Random other bits
vim :help tags-file-format :help cursorhold-example