Difference between SOQL_1 and SOQL_2 in Salesforce

SOQL_1 = [SELECT Id, Active__c,Opportunity__c FROM UID__c WHERE Opportunity__c IN: oppMap.keyset()] ;

SOQL_1 will returns a set which contains all the keys .

SOQL_2 = [SELECT Id, Active__c,Opportunity__c FROM UID__c WHERE Opportunity__c IN: oppMap.Values()] ;

SOQL_2  will returns a List which has all the values .