ASN.1 is short for Abstract Syntax Notation One. It is a notation for
describing abstract types and values. Using ASN.1, one can describe the
format of complex objects by putting together more simpler types.

However, ASN.1 does not specify how these objects are encoded into strings
of ones and zeros. For that, you must use a set of encoding rules. The two
most common encoding rules are the Basic Encoding Rules (BER) and the
Distinguished Encoding Rules (DER). The only difference between BER and DER
is that there are multiple ways to encode objects in the BER, but the DER is
a subset of the BER such that there is only one possible way to encode each
object.

Kerberos 5 uses ASN.1 and the DER to encode and decode all of the Kerberos
protocol messages. Unless you are planning on adding to the Kerberos
protocol itself, you don't really need to worry about ASN.1 at all.

If you wish to learn more about ASN.1, I would suggest reading:

   * Burton S. Kaliski Jr., "A Layman's Guide to a Subset of ASN.1, BER, and
     DER"
     <ftp://ftp.rsa.com/pub/pkcs/ps/layman.ps>
   * Brian Tung, "ASN.1: Wherefore Art Thou?"
     <http://www.isi.edu/~brian/security/asn1.html>