Scott Sumner asks  what aggregate demand (AD) is:
So what do [economists] think AD is? ... What is held constant along a given AD curve? Presumably a given AD curve is supposed to be holding constant things like monetary and fiscal policy, animal spirits, consumer sentiment, etc.
The information transfer model has a simple answer: AD is a source of information that is received by the aggregate supply (AS), with the price level (i.e. the "price" in the AD/AS model) detecting the information transfer.
For a single good we have something like "I would like a pound of bacon for 6 dollars" (demand information) is received by a physical pound of bacon (supply) being sold ... with the price paid "detecting" the information transfer.
Of course, you have people who would buy bacon at 2 dollars or at 10 dollars. This information is "lost" at the supply; in the former case because the sale doesn't happen and in the latter case because the extra 4 dollars is not collected.
This information from the demand includes all kinds of expectations and theories as well. "I think the price of bacon will rise tomorrow to 8 dollars" is also communicated by a purchase of bacon at $6 today -- but again with some information loss. "Bacon is yummy at any price" or "I am making bacon wrapped shrimp for dinner tonight" also comes through, but again with some information loss.
In this way, information received by the supply (IS) is generally less than information received by the demand (ID), or IS < ID. However, IS ~ ID is a good approximation for a functioning market . And only in the case of IS ~ ID do you get supply and demand curves (otherwise you get supply and demand "regions" bounded by the supply and demand curves).
This picture means that IAD is the aggregate of all kinds of economic information, from everyone's future expected path of monetary policy, exchange rates or the size of the tech sector to the current price of commodities and your paycheck. AS is the set of all the things that get made and/or sold because of that information. Typically, IAD ~ IAS is a good approximation in a functioning market, and that gives you AD and AS curves which represent the effect on the price level of a given set of information being sent (AD) or received (AS). (These curves then intersect at some value where AD = AS.)
If you gathered up all the information people put forward at one point in time (think of it as one grand "specification" for an economy ... like an engineering spec) and then varied the amount of stuff actually produced by the economy, that would trace out the "AD curve". The price level falls as you produce more stuff to that paricular specification. In that sense, you can say the entire curve is "the" AD ... at one point in time. This answers Sumner's question about what is held constant along an AD curve. 
Now maybe this is just my opinion, but I think this paints an extraordinarily clear picture of what AD is.
 This post is an adaptation of my comment on Sumner's blog.
 I made the argument once before that maybe ID ~ IS is a condition required to have a sensible theory of macroeconomics.
 For another take, from the perspective of thermodynamics, see here. That specification is the "demand bath" analogous to a heat bath in isothermal processes.