Filtering the result of the find command, so that it returns only directories

Yes, the -type d option is used for this.

For example:

$ find /boot -type d
/boot
/boot/grub
/boot/grub/locale
/boot/grub/fonts
/boot/grub/i386-pc

Here's the relevant section of the man page:

   -type c
          File is of type c:

          b      block (buffered) special

          c      character (unbuffered) special

          d      directory

          p      named pipe (FIFO)

          f      regular file

          l      symbolic link; this is never true if the -L option or the
                 -follow option is in effect, unless the symbolic link  is
                 broken.  If you want to search for symbolic links when -L
                 is in effect, use -xtype.

          s      socket

          D      door (Solaris)

As an addition to Zero Piraeus' answer, if you want to include symlinks that resolve to directories:

• with GNU find:

   find . -xtype d
  

• POSIXly:

   find . -exec test -d {} \; -print
  

  which you can optimise to

   find . \( -type d -o -type l -exec test -d {} \; \) -print
  

If you want to follow symlinks when descending the directory tree, you'd do:

find -L . -type d

which would report directories and symlinks to directories. If you don't want the symlinks:

• with GNU find:

   find -L . -xtype d
  

• POSIXly:

   find -L . -type d ! -exec test -L {} \; -print
  

With zsh:

print -rC1 -- **/*(ND/)   # directories
print -rC1 -- **/*(ND-/)  # directories, or symlinks to directories
print -rC1 -- ***/*(ND/)  # directories, traversing symlinks
print -rC1 -- ***/*(ND-/) # directories or symlinks to directories,
                          # traversing symlinks