In Joomla!, a Menu is a set of menu items used for website navigation. Each menu item defines an URL to a page on your site, and settings that control the contents(articles, category(ies) lists, taggged items, etc.) and style(module(s), layout) of that page. Additionally, each menu has a Menu Type, these are shown in the menu manager, see Adding a new menu. The menu type will actually be the Unique Name or alias that will be used to create human-readable URLs if Search Engine Friendly URLs are turned on. Menu items can have any number of subitems.

A Menu isn't automatically shown on any page. You need to create a menu module to show the menu on all or some pages. Each menu can be shown by one or more modules (making it possible to show the same menu in different positions). It is also possible to create Split menus.

A Menu that isn't shown by any module is usually called a hidden menu. Hidden menus can be used to create URLs that are not visible on any page.

Reference: http://docs.joomla.org/Glossary

Module Class Suffix is a parameter in Joomla! modules. It is set Module: [Edit] screen under Advanced Parameters. Setting this parameter causes Joomla! to either add a new CSS class or modify the existing CSS class for the div element for this specific module.

When Joomla! generates a module, it automatically create a CSS class called "moduletable" to allow styling of the module - for example,

<div class="moduletable">

To create a new class, enter the parameter with a leading space. For example, entering a space plus "myNewClass" will create a new CSS class called "myNewClass". The HTML will be changed to

<div class="moduletable myNewClass">

To change the name of the existing class, enter in the parameter without a leading space. For example, entering "_mySuffix" (no leading space) will cause the HTML to changed to

<div class="moduletable_mySuffix">

Generally, it is recommended to use a leading space to create a new class. This way, CSS styling for this module that uses the standard class names will continue to work. You can use the new class name to add any desired styling to the module without needing to re-create all of the existing CSS code. Note that, if you create a new class name, make sure it has a unique name and doesn't conflict with any existing class names.

Reference: http://docs.joomla.org/Glossary