SIP trunk configuration

Hi All,
Im new to freepbx, but much interested to know in detail.

Can any one explain about the sip trunk and its uses? And also the basic configuration of the sip trunk in the freepbx ?

It would be a great help if you could provide with screenshots.

thanks in advance

You should start reading the FreePBX wiki to get started with FreePBX.
That explains its various functions.