What is sObject .....can anyone give deep meaning of it 

List<Account>Acl = new List<Account>([Select Id,name 

from Account limit 10 ]);

 Map <Id,Account> mpn = new map<Id,Account>();

 for (Account a : acl)
 { mpn.put(a.Id,a);
 }
 system.debug(mpn.get('0019000000PDZvkAAH')); and i should not cal the contact through dot operator ie (mpn.get('0019000000PDZvkAAH').contacts)

 can any one suggest any idea

my code is

Map<Id,Account> newmp = new map<Id,Account>([Select Id,Name from Account limit 10]);
for(Account acc : newmp.values())
{
string codesId = newmp.get('United Oil & Gas, UK');

system.assertEquals(codesId,'0019000000PCwb9AAD');
system.debug('account:'+codesId);
}

I Have a big confussion in using lists and set  in triggers can any one guide me when to use list and sets in trigger

List<Account>Acl = new List<Account>([Select Id,name 

from Account limit 10 ]);

 Map <Id,Account> mpn = new map<Id,Account>();

 for (Account a : acl)
 { mpn.put(a.Id,a);
 }
 system.debug(mpn.get('0019000000PDZvkAAH')); and i should not cal the contact through dot operator ie (mpn.get('0019000000PDZvkAAH').contacts)

 can any one suggest any idea

my code is

Map<Id,Account> newmp = new map<Id,Account>([Select Id,Name from Account limit 10]);
for(Account acc : newmp.values())
{
string codesId = newmp.get('United Oil & Gas, UK');

system.assertEquals(codesId,'0019000000PCwb9AAD');
system.debug('account:'+codesId);
}