This is an oversized DL (1/3 A4) envelope used mostly by the mailing industry for mail inserting equipment.
This is the envelope size for A5 paper or A4 folded in half.
This is the size envelope used for A4 sheets to be inserted without needing folding
BRE (Business Reply Envelope or reply paid envelope)
These envelopes are used to allow people to return or respond to a mailout without having to pay for postage. A special account is required from Australia Post, which then charges the postage back to the original company.
An insert usually refers to an article that either does not require or is already folded to be inserted into an envelope.
This is a method of mail sorting, designed to separate the mail based on letters geographic location. Australia Post provides discounts over standard postage rates for complying with their presort lodgement requirements.
DPID Barcode (Address Barcode)
DPID stands for delivery point indicator, and refers to the exact position of a property address, geographically. The barcode is a font which translates the DPID into a scannable series of lines. This allows Australia Posts systems to sort the mail faster than having to scan the text address.
This is a bulk mail service Australia Post provides were one lodgement contains over 300 correctly addressed letters. Again, there are some requirements to the address layout and lodgement procedures.
The terminology in the printing industry could fill websites all by themselves, so rather than scratch the surface, here is a link to a very thorough list.