When you create a site based on the Records Center site template, the site contains a record routing list with a default "catch-all" routing item, and a document library to hold any documents being sent to the Records Center.  You can send documents to the Records Center by email, or manually using the "Send To..." option on the context menu in a document library.  To make the right option appear in that context menu, you'll need to use SharePoint Central Administration to set up a connection to your Records Server.

When you set up that connection, you need to know the URL of the web service that's invoked to submit documents to the Records Server.  For the default Records Center site, it'll be something like...

http://server/Records/_vti_bin/OfficialFile.asmx

...and this shows the third way to submit documents to the records server; you can invoke the web service from any client-side code.  Perhaps you might invoke it from an Add-in for one the Office applications, or perhaps from your own rich-client application.

What you might not realise is that this web service is also present in all SharePoint 2007 sites.  You don't have to use the Records Center site template to get it.  Indeed, your root site will have it at...

http://server/_vti_bin/OfficialFile.asmx

To set up routing for your documents, first create a library in your Records site (whether that's the Records Center or not) to hold your documents long-term, i.e. when they become records.  Next, create a routing item, the name of which should be the same as the content type of documents you want to be routed by this item.  If you want this routing to apply to multiple content types, you can specify them all in a /-separated list under "aliases" in the routing item.

Any document library can have an information management policy applied to it, but it seems particularly relevant for the libraries used in your Records site.  You can define expiration, auditing, etc., including defining a workflow to action when the document finally expires.