An abstract is a summary of a research write up, which is usually read before the main document. Searching sites such as Google Scholar allows you to read the abstract of a study, when often the full text is only available if your institution subscribes to the particular journal if you pay a one off charge.
In printed journals, an abstract appears at the beginning of every article. It is therefore a way to let readers know enough about a study to decide whether it will be useful to them to read on.
|Psychology Journals - image source: http://bit.ly/11xDy4g|
When writing an astract, it is important to remember that it is a summary of the whole report, so it doesn't just include aim and background, but things like results and conclusions too. Here is an example from the well-known Loftus and Palmer study of eyewitness testimony:
Two experiments are reported in which subjects viewed films of automobiled accidents and then answered questions about events occurring in the films. The question, “About how fast were the cars going when they smashed into each other?” elicited higher estimates of speed than questions which used the verbs collided, bumped, contacted, or hit in place of smashed. On a retest one week later, those subjects who received the verb smashed were more likely to say “yes” to the question, “Did you see any broken glass?”, even though broken glass was not present in the film. These results are consistent with the view that the questions asked subsequent to an event can cause a reconstruction in one's memory of that event.
As can be seen, the abstract gives an overview of the experiment/experiments in the report and certain key details. Things you might include in an abstract include: aim, background, method, sample, hypotheses, procedure, findings, conclusions. However, it should be short - not more than 150-200 words. Overall it should be understandable as a stand-alone piece.
As a student, the exact details of what you must include may depend on the marking scheme used by your institution or exam board, so that should be checked carefully. For submission to publication, refer to previous examples to get an idea of the house style of the particular journal.
The abstract always goes at the start of your write up. However, because it is an overview, it is often a good idea to write it last.