A protocol is a set of rules for a specific pourpose. They decide how the data packet will be broke up and what each 1 and 0 is there for. Some protocols are simplY in place to assist other protocols.
Usaly a protocol decide how a header and or trailer of a packet will be set up. The center (payload) is the data you want(MOST OF THE TIME). ATM for example is broke down into octets. an octet is 53 bits or bytes I forget now. Anyway the Payload is all the back end. All the over head is in the header.
They are also a set of rules that state how that packet is to be handled and by what, and on what level of the OSI modle they will be handled.
Check it out... http://onenetworks.comms.agilent.com/forms/IntAdv_Poster.asp
Follow that link, fill out the form with your real address. They will send you a huge poster of the main networking protocols...
Sun NFS Protocol suit
IBM protocol suit
ect ect ect....
It is awsome. It is a map of all these protocol suits broken down across the OSI modle so you can see what gets done where.
That site also has posters for free on Cellular protocols
Anatomy of a PCS signal
Anatomy of a GSM and GPRS signal
Telecom networking communications
Anatomy of IN/AIN signal
It was a truley good find for me. They do send the posters our It took about 2 weeks.
It will give you pleanty to read up on on http://protocols.com