Active directory contains forest, tree, domain and its 
child domain. Child domian is comes under parent domain and 
it shares the name space, its names space append the parent 
domain name.  Addtional domain controller is the copy of 
main domain controller and its for load balancing and fault 
tolarance.

the main difference between main domain and child domain is 
that.the main domain have a right of enterprise admin and 
domain admin and schema admin. but child domain dont have 
this right.

The difference between main domain and child domain.Child 
domain is come under the main domain.Additional domain is 
for only using the load balancing and fault tolerance.

logical diagram of active directory is
1.forest
2. tree
3.domain
4. o.u (organisational unit)
and the physical structure is
1. sites
and 2. domain controller

the main difference of child and additional domain is
additional domain is the backup domian which is used for 
load balancing and for fault tolerance and domain is the 
subdomain. and both will be having the different roles.