How do I search for a command I don't know the complete name of?

This will list all man pagenames and short descriptions containing the text "disks". Note that this will not return anything for scripts or programs that do not have a manpage but every program supplied with the Ubuntu distro should have one...

man -k disks

OR...

apropos disks

For example...

 root@LX02:~  apropos disks
cryptdisks_start (8) - wrapper around cryptsetup which parses /etc/crypttab.
cryptdisks_stop (8)  - wrapper around cryptsetup which parses /etc/crypttab.
mtools (1)           - utilities to access DOS disks in Unix.
smartctl (8)         - Control and Monitor Utility for SMART Disks
udisks (1)           - udisks command line tool
udisks (7)           - Storage Management
udisks-daemon (8)    - udisks Daemon

The easiest way is to inspect the "program starter" using the menu editor:

• right-click on the menu button in the taskbar;
• select Edit Menu - this will open the menu editor.

Now, you can choose the category at the left and the application at the right (Entries). Just select the application with a single left click, open the context menu with a right-click and select Properties. A small window will appear, showing (among other information) the command line.

Those program starter files usually reside in /usr/share/applications, so another approach would be to check the *.desktop files in that directory. If you're not sure about the name of the *.desktop to look for, do a grep from inside the /usr/share/applications directory:

grep gThumb * | grep 'Name='

will give you something like

gthumb.desktop:Name=gThumb
gthumb.desktop:X-GNOME-FullName=gThumb Image Viewer
gthumb-import.desktop:Name=Import with gThumb
gthumb-import.desktop:X-GNOME-FullName=gThumb Photo Import Tool

Now, you can just use less gthumb.desktop to inspect the details of that program starter.

Another way would be to use the apropos command to search a certain keyword in the man pages.