If you don't care where the rules come from, or what they are for, you can skip this section.
I happen to work in the telecommunications industry, and I have experience with ASN.1 and related standards as defined by the ITU (others know these standards either through the ISO or from RFC's). ASN.1 is used by the GDMO (roughly, ``Guidelines for the Development of Managed Objects'' - there are several ways I know of to decompose that acronym) to describe MIBs (Management Information Bases). The ``Simple Network Management Protocol'' (SNMP) for example works with a MIB describing various aspects of a networked device (like network cards, routing tables, and IP addresses).
Basically, ASN.1 is a way to describe data types in a machine independent way from a text description (something like XDR - the eXternal Data Representation used by ONC/RPC. Go and find out yourself what the latter is). Vaguely associated with ASN.1 are sets of encoding rules, such as BER (or Basic Encoding Rules) which describe how to actually create external representations of data. There are other encoding rules (e.g., FER), but you've had enough acronyms for now. ASN.1 is really a very powerful tool - you may want to learn more about it on your own.