⌘ -- Command
⌥ -- Option/Alt
⌃ -- Control
⇧ -- Shift
Therefore, if you see a hot key such as "⌥⇧⌘V", then this means to press Option-Shift-Command and then press "V" to invoke the command.
While the menus by default will show a list of basic commands, they also can be used to find some of the hidden and obscure commands and the keyboard shortcuts that accompany them. To see this, just activate a menu and then press various combinations of the modifier keys (Shift, Control, Option) to see what new commands show up in the menus. For instance, if you open the File menu and press the Option key, you will see the Command-W shortcut for closing windows change to be "Option-Command-W" for closing all windows.
In addition to browsing through menus and trying different modifier keys, OS X has a number of shortcuts that may not be available in the system menus, such as those for taking screenshots, invoking the screen zoom functions, or changing the behavior of the Dock so it hides or shows itself (among many other system settings).
In these cases, you can look up the various keyboard shortcuts by going to the Keyboard section of the system preferences and selecting the Keyboard Shortcuts tab, where you can select various system areas and see lists of the shortcuts that have been enabled for those areas. In this system preferences pane you can enable or disable shortcuts, change them to something different, orfor menu items that do not have an assigned shortcut.
Finally, while these resources within OS X are useful, especially for finding shortcuts within the context of menus, Apple does have a knowledgebase document that contains a listing of practically all the shortcuts that are included in OS X. If you need a resource to pin up next to your computer, or at least bookmark to look up shortcuts you might find useful, then this list is handy to have.
Overall, shortcuts in any OS are exceptionally useful, and knowing where and how to look for them in OS X will help make your workflow more efficient.