LPD is an old standard, IPP is newer.

Ubuntu uses the Common UNIX Printing System ("CUPS") to handle printing. CUPS uses the Internet Printing Protocol ("IPP") as the basis for managing print jobs and queues. Other protocols are also supported (LPD, SMB, AppSocket a.k.a. JetDirect), some with reduced functionality.

LPD/LPR is still commonly used and works quite well but it doesn't provide much control for users on the printer settings per print job.

Both the LPD and JetDirect/AppSocket protocols can be used over the Internet today, however neither of these protocols provides authentication services, access control, and all of the document management and formatting (including printer-specific commands) must be handled by the machine sending the document.

IPP is preferred as it uses bidirectional communication which gives you more feedback and control.

Some printers may not support IPP.