You want all "not found" pages to go to some other page instead, such as the front page of the site, so that there is no loss of continuity on bad URLs.
Use the ErrorDocument to catch 404 (Not Found) errors:
ErrorDocument 404 /index.html DirectoryIndex index.html /path/to/notfound.html
The recipe given here will cause all 404 errors every time someone requests an invalid URL to return the URL /index.html, providing the user with the front page of your web site, so that even invalid URLs still get valid content. Presumably, users accessing an invalid URL on your web site will get a page that helps them find the information that they were looking for.
On the other hand, this behavior may confuse the user who believes she knows exactly where the URL should take her. Make sure that the page that you provide as the global error document does in fact help people find things on your site, and does not merely confuse or disorient them. You may, as shown in the example, return them to the front page of the site. From there they should be able to find what they were looking for.
When users get good content from bad URLs, they will never fix their bookmarks and will continue to use a bogus URL long after it has become invalid. You will continue to get 404 errors in your log file for these URLs, and the user will never be aware that they are using an invalid URL. If, on the other hand, you actually return an error document, they will immediately be aware that the URL they are using is invalid and will update their bookmarks to the new URL when they find it.
Note that, even though a valid document is being returned, a status code of 404 is still returned to the client. This means that if you are using some variety of tool to validate the links on your web site, you will still get good results, if the tool is checking the status code, rather than looking for error messages in the content.