Which objects are displayed when i click on related lists , during ediing a page layout on any Object? Some times I don't find the objects I am looking for when i click related lists when editing a page layout of a object in Salesforce
The related lists that are available for page Layouts on Objects depends on the object. Some lists are available to most objects like Activity History while others are specific to the object for example Account Teams list on Account object. For custom objects if there is a master-detail relationship usually the detail object is available as a related list for the master object. Do you have a specific object that your are looking at?
Generally speaking, any time there is a relationship field on another object that points to the object whose page layout you're editing, the object with the relationship field will be available as a related list.
For example, the Account object will have Contact, Opportunity and Case related lists available (among others), because each of those objects have a lookup or master-detail relationship field pointing to the Account object.
There are also some special related lists, like Notes & Attachments and Activity History. These appear on most standard objects that support activities, and they'll appear on custom objects if you specify when you create an object that you want to allow activities or notes/attachments.
If an object you're looking for isn't showing up, check that it has relationship field pointing to the object you're editing.