Very good question. I will like to augment the points that jijoe has highlighted.
First of all lets look at Signalling from a Telecommunication perspective. Signalling is all about the required steps that will be used to Initiate, Maintain and Clear Down a Telephony (Voice) or Data session (e.g. Internet).
The required steps is basically the protocol that will be used and this contains the rules and syntax that will be used between co-operating Systems and Entities. An analogy that I often use to explain what what
is meant by a protocol is the basic idea of a handshake. When you meet a person you extend your hand and you
shake their hand. Afterwards, you communicate. One party (A) talks whilst the other party (B) listens and when appropriate the other party (B) will respond. There is an agreed protocol that is in operation.
For Telecommunication (Systems) Signalling will use an agreed protocol: e.g. SIP, Signalling System Number 7 (SS7), R2, MFC, Loop_Disconnect, AC9, DC10, etc..
VBR/ Wallis Dudhnath