I didn't understand Packet Classifier Element

Hi there.

Please, could someone explain to me how Packet Classifier Element works?
For example, I want to understant how this command knows what is the arp
request, response and IP packet.

c0 :: Classifier(12/0806 20/0001,
                 12/0806 20/0002,
                 12/0800,
                 -);

I didn't understant the offset/value definition.

Thank you.

Emerson Barea