Depending on input impedance, a series resistor with 2 diodes (one connected to Vcc to protect input to Vcc+0.7V, second connected to 0V to protect input to 0V-0.7V) will provide suficient protection for ADC input.
If you use an opamp as a buffer then you will need to protect its input in the same way as described in the previous post.
There are no reasons to employ opamps unless you would like to change input signal's range, increase input impedance etc..
Here you will find a lot of info on theory, selection amd configuration of opamps..
(buffers, invering and non-inverting).. **broken link removed**
R- limits the current
Zener diode prottect against voltages greater than Vz. Vz is slightly greater than Vdd.
Schotky diode protect against negative voltages <-0.2V