Q) what is custom content type? and what for is it good?
A) Assume you know about the Content Type basically a content type is a reusable collection of settings that are applied to a certain category of content. Content types enable the management of the metadata and behaviors of a document or item type in a centralized, reusable way. Also used for content classification (Taxonomy).Above text taken from the following links and it also discuss about creating Custom Content Typehttp://msdn.microsoft.com/en-us/library/cc161067.aspx
Check the below PDF for more information about the Custom Content Typeshttp://download.microsoft.com/documents/uk/technet/downloads/technetmagazine/may_08/26_36_Stnd%20D%20ContentTypes_des5.pdf
Creating and Using Content Types in Windows SharePoint Services 3.0 by ted pattsion http://go.microsoft.com/?linkid=5519358
This blog discussion about creating Content Type and association with a workflowContent Types: Part 2 - Creation and Association of Workflows and Document Templates (Republish)and a very good discussion about Content Type can be found herehttp://blogs.msdn.com/jamesway/archive/2008/01/22/content-types-the-little-engine-that-could.aspx