Hi Shruti,
Application tracking system is a software that allows you to maintain a database of applicant and job info. This information is useful for finding matches between openings and applicants. The software automates all steps in the staffing and hiring process, saving one time and money..(need not be the case always). It is useful for compliance, reporting and process efficiency. It allows all the necessary players across the enterprise to participate and contribute in streamlining the company's business process.
An ATS (Applicant tracking system) lets you create, post, update, and remove jobs instantaneously, so that the job listings are always up to date. You won't depend on a webmaster to keep your job site current - you'll be able to do it yourself. Additionally, it receives and tracks resumes, evaluates candidates and communicates with them automatically.
The only drawback being that with all the tasks involved in recruiting, an ATS can get complicated - and costly - very quickly.
This is how it works.....
Candidates, whether applying through the corporate recruitment website or a third-party job board, will be directed to a one-page online application form for that job. This application collects the most important data for prescreening via a quick and easy interface. All applicants normally receive an automatic e-mail from the software's recruiter confirming receipt.
Most big IT firms have an ATS...though..I haven't come across any popular ones to name. Perhaps, somebody else could help you in this matter.
All the best!