Can Anyone Explain Post Install and Uninstall Script in Salesforce?

Hi,

Post Install 

Example -

global class PostInstallClass implements InstallHandler {
global void onInstall(InstallContext context) {
if(context.previousVersion() == null) {
Account a = new Account(name='Newco');
insert(a);

Survey__c obj = new Survey__c(name='Client Satisfaction Survey');
insert obj;

User u = [Select Id, Email from User where Id =:context.installerID()];   
String toAddress= u.Email;
String[] toAddresses = new String[]{toAddress};
Messaging.SingleEmailMessage mail =
new Messaging.SingleEmailMessage();
mail.setToAddresses(toAddresses);
mail.setReplyTo('support@package.dev');
mail.setSenderDisplayName('My Package Support');
mail.setSubject('Package install successful');
mail.setPlainTextBody('Thanks for installing the package.');
Messaging.sendEmail(new Messaging.Email[] { mail });
}
else
if(context.previousVersion().compareTo(new Version(1,0)) == 0) {
Survey__c obj = new Survey__c(name='Upgrading from Version 1.0');
insert(obj);
}
if(context.isUpgrade()) {
Survey__c obj = new Survey__c(name='Sample Survey during Upgrade');
insert obj;
}
if(context.isPush()) {
Survey__c obj = new Survey__c(name='Sample Survey during Push');
insert obj;
}
}
}

Post Uninstall 

Example how to test uninstall script-

@isTest
static void testUninstallScript() {
Id UninstallerId = UserInfo.getUserId();
List<FeedItem> feedPostsBefore =
[SELECT Id FROM FeedItem WHERE parentId=:UninstallerId AND CreatedDate=TODAY];
Test.testUninstall(new UninstallClass());
List<FeedItem> feedPostsAfter =
[SELECT Id FROM FeedItem WHERE parentId=:UninstallerId AND CreatedDate=TODAY];
System.assertEquals(feedPostsBefore.size() + 1, feedPostsAfter.size(),
'Post to uninstaller failed.');
}

Hope this helps.