I combined all my parts to get a single and deleted history. I am confused at this level. When i combine and delete history, combined individual parts still keep their own histories? What does grouping help ? Combine or group? whats the difference... 
Thats why i think i need a source to learn the aspects. I know the techniques to visualise but like a 3 year old kid drawing good pictures but in a messy way....
By combined, do you mean an actual poly mesh combine? Eg, Edit Mesh>Combine? Just grouping the objects, and deleting the history on that won't have any effect.
Group means a group of objects. It is useful for scene organization, and rigging down the line, as you can nest them, and each nested group can have a different constraint. It is represented like this:
Code:
+-- Node1 Group1
+-- Node2 >> |
+-- Node3 +-- Node1
|
+-- Node2
|
+-- Node3
Combine means take a group of objects, and stuff all the geometry data into one node. Hence it would look like this this:
Code:
Node1
Node2 >> Node4
Node3
You can perform normal polygonal operations on a combined mesh. You can't on a group of MESHES
This video should give you a deeper understanding on what Maya is doing behind the scenes. https://agentfx.blogspot.com/2010/08/...a-nodes-1.html
Imagination is more important than knowledge.