Arguments are input. While they often come from the URL, they don't always so please don't be shocked when they don't. Each display time may have its own source for arguments. Block displays have no source of arguments at all; they cannot pull arguments from the URL, and often require use of the PHP code default in order to get arguments. In general, arguments are used to filter the view, and in that sense they have a very close relationship to filters, but that isn't necessarily true for every argument. Arguments can be used for any purpose, really; the extent of what the argument does is up to the developer of the argument, but the arguments that come with Views are almost entirely filters.

Argument defaults

Ignore: The argument is removed from the view as though it weren't there and all results will be displayed.
Hide view / Page not found: The view will remove itself entirely if the argument is not present; for a block this means it simply won't a appear. For page views the View will return a 404 and present a "Page not found" error.
Display empty text: The value of the empty text will be displayed.
Summary: The view will attempt to display a summary of arguments available, based upon the view, and provide links to the view with those arguments. Summaries have their own style handlers as well as options. The default summary style simply displays a list of arguments with a count of entries found per argument. This special mode is a very powerful part of Views.
Provide default: If no argument is given, a default argument can be selected. The method of choosing the default argument is selectable and pluggable. By default, Views comes with 2 methods to choose the default argument: Fixed entry, where you simply enter the argument, and PHP code, where you can enter a snippet of PHP code that returns the argument.

Wildcards

All arguments have a 'wildcard', which is an argument that means to use all values. In practice, it is the same as 'Ignore' above, where the argument is simply removed and the view is created without the argument. The wildcard title is used in titles and breadcrumb trails.

Argument validation

Arguments can also have validators, which are pluggable systems used to ensure that arguments fit within certain parameters. When a validator is chosen, it may provide some settings for the validator, including the action to take if an argument is presented, but it fails to validate. These actions are generally the same as the default actions above, excluding the ability to provide another default. If a view provides a menu option, such as a tab, if the argument does not validate the tab will not appear. This sytem can have other validators plugged in; by default, Views provides:

Node: Ensure that the argument is a valid Node ID. You may select which types of node the validator will accept.
PHP Code: You may enter arbitrary PHP code, similar to the php block visibility code, to determine if the argument is valid or not.