What is an object identifier? Object identifiers are, basically, strings of numbers. They are allocated in a hierarchical manner, so that, for instance, the authority for "1.2.3" is the only one that can say what "1.2.3.4" means. They are used in a variety of protocols. The formal definition of OIDs comes from the ITU-T recommendation X.208 (ASN.1), which is available from the ITU (if you have your checkbook handy). The definition of OID is in chapter 28; the assignment of the "top of the tree" is given in appendixes B, C, and D. The encodings - how you can transfer an OID as bits on the wire - is defined in X.209.
