The libraries are divided into several packages.
GHC/GHCi users note: most of the time, all the packages are available without the need to use any additional command-line options. The only time when this is not the case is when linking a program on the command line, without using the --make option: in this case you must list all the packages to link to by adding the -package P flag (where P is the package name) to the command line for each package. See the User's Guide for more details.
Hugs users note: Hugs currently provides modules from the base, haskell98, network and haskell-src packages. These modules are always available, as a single hierarchy; Hugs does not use the package subdivision.