Hi Madhu,

Territory management is account sharing system that grants access to accounts based on the characteristics of the accounts. It enables your company to structure your Salesforce data and users the same way you structure your sales territories.
If you want to use territory management in Apex then you should read this document for limitations and territory functionality in Apex.

Relationships among Territory, Account, Opportunity and User objects:
a) A territory can have unlimited number of users, and a user can be assigned to an unlimited number of Territories.
b) A territory can have unlimited number of accounts, and an account can be assigned to an unlimited number of territories.
c) An Opportunity can only be assigned to a Territory.

Territories in Opportunity object:

Problem:
If you want to access the territory field of Opportunity in Apex you can access it easily. Because the field Territory on Opportunity object is exactly present in Opportunity object.

Solution with Example Code:
/*
Author: @Abrar Haq
Purpose:
    (a)To populate Parent Territory Name and Territory User Name of assigned Territory of Opportunity.
*/
trigger OpportunityTrg on Opportunity (before insert) {
    if(Trigger.isBefore){
        /* Declaration of collection data types */
        Set<Id> setOfTerritoryIds = new Set<Id>();

        for(Opportunity oppty : Trigger.New){
            if(Trigger.isInsert){
                if(oppty.TerritoryId != null){
                    setOfTerritoryIds.add(oppty.TerritoryId);
                }
            }
        }

        /*
            (b)To populate Parent Territory Name of assigned Territory of Opportunity.
        */     
        if(setOfTerritoryIds.size() > 0){
            Map<Id, Territory> mapOfTerritory = new Map<Id, Territory>([Select t.RestrictOpportunityTransfer, t.ParentTerritoryId, t.OpportunityAccessLevel, t.Name, t.MayForecastManagerShare, t.Id, t.ForecastUserId, t.Description, t.ContactAccessLevel, t.CaseAccessLevel, t.AccountAccessLevel From Territory t Where Id IN :setOfTerritoryIds]);
        }
    }
}


Please see the link below for more info

http://blogatsalesforce.blogspot.com/2013/05/territory-management-in-apex.html

Best Regards
Naga Kiran