Here are a few handouts that I've put together over the years to provide background information about depression.  The development & maintenance diagram is probably the handout here that I use most - both to explain issues about depression and also for many other psychological disorders as well. 

Development & maintenance of distressed states - I use this Powerpoint diagram a lot when discussing with people why they are in a distressed state.  The diagram applies to depression but it also applies to nearly all other distressed psychological states as well.  It can be helpful in highlighting the importance of maintaining, precipitating and vulnerability factors.  I also point out that therapeutic gains can be made working with all three of these general sets of factors - for example, emotional processing work for past experience (both precipitating and vulnerability factors) and more standard cognitive-behavioural approaches for maintaining factors. 

Why do we get depressed & what can we do to get better? - I print out this pair of Powerpoint slides as a two-slides-to-a-page handout.  It combines the development/maintenance diagram (see above) with an introduction to an evolutionary psychology view of depression.  This latter can help to destigmatise the depressive state and make further sense of it. 

Diagnosing depression - this two-sided handout gives diagnostic criteria for DSM-IV diagnoses of depression, dysthymia, mania & hypomania.  It also gives a diagram illustrating different kinds of depression.

Depression, background information - I put this four page handout together quite a few years ago now.  It seems to have withstood the passage of time pretty well, with most of the facts and figures still accurate.

"Vicious circles" model & other ways of feeling/responding - two Powerpoint slides I typically print out as full A4 handout sheets.  They illustrate how behaviours, relationships, body states & thoughts/images can all combine as vicious circles maintaining depression or as methods that combine to encourage recovery.

Antidepressants, timing of response - traditionally people tend to be told that they will have to wait for three or four weeks before they may get much benefit after starting antidepressants.  This isn't correct and these research abstracts highlight that response typically occurs in days not weeks.