Sitecore supports the concept of an alias, a short URL that can provides an alternate and easier way to navigate to a content item deep in the content tree. For information about configuring aliases, see here: http://sdn5.sitecore.net/Articles/Administration/Aliases.aspx.
Aliases impact the content tree in that they implicitly create items in the content tree. For example, a business user can create an alias /Product A.aspx that points to an item deep in the content tree, such as /Products/Category A/Subcategory A/Product A.aspx. Implicitly, a new node under the root of the site has been created. Business users should be careful to avoid explicitly creating content items that duplicate aliases, as unexpected behavior will occur.
There are at least a few limitations to aliases: 1) aliases must always end in “.aspx” and, as such, may not be usable for marketing purposes and 2) aliases can only be children of the site root – the UI for adding aliases prevents users from creating deeper aliases, such as /Featured Products/Products.aspx. The greatest utility of aliases may be for email marketing, where lengthy urls in plain-text messages may be truncated by email programs.
Marketing URLs are a more flexible form of aliases. See here for more information: (http://sdn5.sitecore.net/Scrapbook/Friendlier%20Marketing%20URLs.aspx). Marketing URLs are handled by URL rewriting tools such as ISAPI_Rewrite. Marketing URLs do not need to end in .aspx and can be as deep as you like. Marketing URLs can be mapped to their proper Sitecore friendly url. Other solutions for marketing URLs also exist (such as including logic in your 404 page) and are beyond the scope of this document. The point in mentioning marketing URLs is to provide potentially more useful alternative to aliases that will generally not complicate the administration of your content tree.