What is sub ledger? How is it linked to GL?

A sub ledger is used for tracking individual items and 
transactions separate to the General Ledger (GL). Sub-
ledgers may exist for, but are not limited to Accounts 
Receivable (AR), Accounts Payable (AP), Inventory and Fixed 
Assets etc. Without a sub-ledger individual customers or 
inventory items would require their own GL code making the 
GL unnecessarily large. Sub-ledgers also allow for more 
specific detail to be included in transaction details. Each 
sub-ledger links to the GL by way of a control account 
(e.g. Debtors control the AR sub-ledger etc.). The total of 
the sub-ledger account should equal the balabce in the 
control account at all times. Individual transactions 
should be posted via sub-ledger not directly to the control 
account. Reconciliations should be performed between the 
sub-ledger and control account on at least a monthly basis 
to ensure this is the case.