PLC: Personal Learning Community. I look at a PLC as a group you physically meet with to plan and collaborate. Your PLC can be within your own school or your own district. Some may say a PLC can also include individuals within the same demographic area.
PLN: Personal Learning Network. A PLN can reach individuals all over the world. You create a PLN by meeting individuals at conferences, attending webinars, following people on Twitter and many more ways.
You might wonder (just like a Web 2.0 tool) which is better. Well - both are quite useful/beneficial to everyone involved. You definitely need that PLC because you work together on curriculum mapping, lessons and more. You can use knowledge you gain from your PLN with your PLC.
My suggestion to you - join Twitter and start following education individuals/groups. Start expanding your PLN and share with your PLC! The more connections you make - the more you learn! The best teachers are teachers that want to keep learning!