Distinguished Encoding Rules
Distinguished Encoding Rules or DER is a transfer syntax defined by the ITU in X.690. It is a Presentation Layer protocol which allows applications to exchange information. ASN.1 includes cryptographic extension which allows peer to peer encryption between applications.
DER is the most stringent format of the encoding rules. Slightly more flexible is CER, with BER being the most flexible. ASN.1 also specifies syntax for XER, or XML encoding rules.
Viewing and Editing Tools
Peter Gutmann's asn1dump is the commandl line tool of choice for viewing certificate encodings. Gemini Security Solutions  offers a visual front end to asn1dump.
Objective System offer a visual ASN.1 editor , which allows viewing and editing of a certificate similar to a binary hex editor.