S parameters for Networks anaklyzer ?

Could someone explain to me the meaning S parameter in a network analyzer?
or a link to a resource where it explains

For example her is any measurement for S11 on 13.50MHz.
- what all of it can be read?
- It investigates possible impedance of the measured object to 13,5MHz?
- As the measured object connected?

For a first starting point and the basics about s-parameters, you can google for "agilent an95-1".