Difference between customizing default solution & unmanaged solution?

hi all,

 This is very naive question but I would like to understand the best practices followed in CRM 2011 customization.

What is the difference between customizing a default solution and an unmanaged solution of a fresh CRM 2011 instance? I understand we can define a new publisher information when creating an unmanaged solution and customizing it.

Apart from the above, is it good to create an unmanaged solution and customize the entities or is it good to customize directly on the default solution?

Which one is the best practice to be followed?

Thanks,

kva888