Related List, Related List Label and custom fields? Help!!

Related List, Related List Label and custom fields? Help!!

I'm having trouble understanding the related lists, what they are for, how they map, etc.  I've added custom fields in the Leads module, for example, to associate a Vendor with a lead.  I create the new field using the following

Field Type: Lookup
Label: Vendor
Lookup Type: Vendor Module
Related List Label: ?????

I entered "Products" under the first Related List Label.  Now, when I go the specified vendor, there is a duplicate Products section with the Lead associated.  I've created a few other custom fields that are Lookup fields and I just am not understanding the Related List Label, how to use it properly, how it maps, why it's there, etc.  Can someone help?