Hi Hariharan! Thanks for your question, and welcome to the Target community! The at.js library offers several improvements over the mbox.js library, so you may find this resource on at.js Implementation within the Target Product Documentation helpful.
Since you're new to the Target Community, I'd like to highlight a few threads here on the forum that are packed with resources:
1. Upcoming "Personalization Thursdays" webinar registration links
2. Recordings of past webinars
Also, be sure to register here for this week's webinar on Discovering & Managing Audiences for Maturity
Hope this helps!
Warmly,
Amelia