Hi Smitha,
1) What you're looking for is multipass, here's a document on the topic that you can take a look at : http://docs.shopify.com/api/uiintegrations/multipass
2) You can have Shopify send a Webhook on certain events (document here : http://docs.shopify.com/api/webhooks/using-webhooks), or you can make calls from the app to your store for the information that needs to be brought into your other system - it's up to you!
If you ever want to give us a call to talk a little bit about your plans, we're reachable 24/7 by phone or live chat - http://docs.shopify.com/support