Whats ISR mean?

I have an idea of how i can write this code in a few simple lines but i dont know how to use interrupts at all. I assume i have 2 interrupts. If one of them goes low then that is the zero-crossing and the code will goto the PWM area. If the second interrupt pin (serial) changes then do i run serin toget the data or wouldnt that work?

2 main things i need to know at the moment.
Which chip do i need?
How do i detect interrupts?

BTW. Im buying all my parts from http://www.rapidonline.com so it would be nice if i could get the chip from there. I dont think that last one i found is available from them.

Edit: Do the PIC codes mean anything? Like 16F = 20 pin, 18F = 40 pin etc. Or do they just increment with each new chip?