level 1 is the level you are at by default (before you enable). Level 1 allows you to do show commands to see stats, etc. Most show commands can be ran here - even ones that don't show up via the ? mark.
enable without any level # goes to level 15 by default. This gives you full access.
levels 2 through 14 don't have any differences by default. You can configure commands, configuration items, etc for these lower privilege levels however. This way you can have some users with privilege level of 8 for example and they might have rights to clear counters, run a certain debug, or some other task.
Level 0 is the lowest level and this doesn't allow hardly any commands by default. You need to set the commands down to priv level 0 that you want users to see. Be careful with this level as there may be some commands within IOS that need level 1 to run by default (example is show refuse-message).
