IP Phones

I just saw an article on ' IP Phones' on the net, while i was searching about Voip.
I have a simple question, 'Can IP Phones used to make calls to PSTN Networks?'
If 'Yes', where is the connection between IP and PSTN made?

IP Phones> How does it differ by normal VoIP Phones?

There has to be an internet to PSTN network port. Several companies provide this service in various cities. Most charge a fee.

what do they use/components they use? how it converts the voice into packets? and packets into voice at other end?

They use Voip gateways and ATA (analog telephone adapters). Neither of which are items you would want to attempt to build yourself. Spend a little time reading up on Voip and how it works. Places to start voip-info.org - voip-info.org Cisco Systems, Inc **broken link removed**