There are basically two kinds of information (that sometimes overlap). One kind is evidence than an event occurred some time in the past. The second is an organized thought (for example the words on this page), although partly the transmission of this type of information consists of evidence that a certain thought occurred in the past. Here I will give an account of the transmission of both these kinds of information.
First let me address the simpler type of information, that of evidence for some event in the past. For an event B to be evidence for event A it must be the case that A is a cause of event B. This is not to say that event B gives us certain knowledge that A occurred. In fact often it is the case that while A may have caused B there are many other events that could also have resulted in B. However we still say that B transmits information about A, because taken together with other events we can establish that A definitely had to occur, or at least that it is very likely that A occurred.
For example let us consider a tree outside the window. How does the information that there is a tree outside get to me? Well most immediately I have the visual impression of a tree. That visual impression is most likely caused by light striking my eye (although in theory it could be a hallucination). That light in turn is most likely caused by the existence of a physical tree (and not a hologram). Thus the tree itself is a cause of my visual impressions (indirectly), and more importantly it is the most likely cause for them, giving me both the information that a tree is there and good reason to believe this information to be accurate.
Although everything that can be known to exist interacts casually with the world, it is still possible for information to be lost. A trivial case is when the only result of an event is to create a particle moving away from us at the speed of light. Although this particle does technically carry information about the event we will never be able to observe it, and thus the information is lost to us. A more typical case though is when the information is lost due to “noise”. For example let us say that event A causes event D. It is also possible however that event D though could have been caused by event C. Now let us assume that nearby to A event B has caused event E. Once again however, C could have been the cause of E as well. Thus from looking at events D and E we will conclude that event C took place, when really it is a combination of A and B, and thus the information that A and B occurred has been lost. In situations such as this the other possible causes of an event (the noise) have outweighed the real cause, making the real cause seem no more likely, or possibly even less likely, than some other combination of events. Although technically the information about the real events still is there it is no longer discoverable.
Now we can build on this foundation to discuss the transmission of organized thoughts, which is what is more commonly thought of as information. One condition for an organized thought to be transmitted is that the thought must have a casual effect on the world. For example my thoughts now are being written down here, and thus have a casual effect on the computer, and later they will have a casual effect upon my readers. However there is also a second condition, which is that when the recipient receives the information that similar mental models to those entertained by the author must be invoked in their minds.
To see why this extra condition is important consider the following example: a man in a foreign country writes a book, that he ships to you. Unfortunately you cannot read his language, but even so when you receive his book some information has been conveyed to you, namely that a man in a foreign country is sending you books (this is the first kind of information). Clearly this is not the same as the information that is the content of the book. However when someone who speaks the language of the author reads the book they will have thoughts corresponding to those the author had when committing the words to print. (For example if I write the words “A man in a red house” I must think of a man in a red house, and if you understand me you also will think of a man in a red house when reading it.) It is not just books that convey this kind of information however; art may be a vehicle for it as well, and in some ways may be more successful since it can cross language barriers.
This type of information is even easier to lose than the first kind. For example small changes in the transmission medium (the events that result from the original) can completely change what a recipient will think (for example painting all the pages of a book red will destroy the information contained within it). It is also possible that people will forget how to interpret the writing or the art. In this case the information may be lost without anything happening to the transmission medium itself. It is specifically for this reason that I have separated the first type of information from the second. The first type of information is almost completely independent of the observer, in the sense that it bears the same information no matter who observes it. The second type however is not so independent of people; if no one can understand it the information is effectively lost.