A basic question about AD license, if on my company website, clients makes an account and would be authenticated through AD server, do i need to have the licenses for the clients coming via web. and what all license would come into picture.
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If your users are authenticating to AD, then it is likely that you will need Authenticated SPLA licences for your server, however your best bet is to call MS Licencing and ask them. An even better bet is to call MS licencing more than once and ask different people there, as there answers often vary.
That said, id there any reason why your Web App needs to use AD to store it's user accounts? Using AD user accounts is generally only really done when you need other features of AD, like Exchange or SharePoint Access, if you just need to authenticate users then why not just store user accounts in your database. You'll save your self a lot of money because you will only need an unauthenticated Windows Licence.
YOu need to have one CAL for every client then. UNLESS the client accounts are like web service hosting accounts and are basically for technical means. Depends a lot on the details here. What are "clients", what do they use the system for? (i.e. do they pay to use the system, like web hosting, or is the system only a means to an end, like a web shop like amazon where they pay for goods and not for system access).
Depending on the answer to that question, normal licensing may apply - or you may HAVE TO (!) use the SPLA (Service Provider License Agreement), which is more cheap and paid monthly.
If SPLA does not apply, an internet connector license may be used.