Generally architecture considers the way things are arranged, especially including the ways its parts are connected and things flow between them.
Internet Architecture thus refers specifically to the way the Internet is connected and information flows through it.
This means in particular it considers the protocols that guide the flows, and the physical instructure of channels and routers and middleboxes through which things flow.
Since we have now had several decades of highly succesful experience with the Internet that have led to its use in just about every ended for current thoughts about what is being done on the project, and the wiki is intended for continuing thoughts about various ideas related to the project.
These references (under construction and not yet organized) are generally to papers on the topic.
A basic principle of Internet architecture, which in some opinions is now in need of replacement, is that the endpoints of a connection are generally the best place to implement functionality that needs the cooperation of the endpoints.
This principle is related to another less prominent notion called "fate-sharing". which says that state information about a connection should generally be held at its endpoints.
See Will The Real "End-End Principle" Please Stand Up? for a much better explanation of the two principles and their difference.
In a totally different direction the principle is related to a currently highly-controversial and very political topic called "net neutrality".
Since with UDP traffic there is no connection in the usual sense the whole notion of end2end carries a totally different meaning, if any, in this case.
One topic of interest to me in various ways is what may happen to this principle.