The abstract is a stand-alone summary of the key ideas and results from a paper. Although it normally is placed at the start of the paper, it should not be used as an introduction. Some people may read only the abstract, while those who read the entire paper may skip the abstract. Not every science journal includes an abstract. In those that do, length is variable but is usually no more than 200 words.
The abstract exemplifies the challenge of making your writing precise but also concise. It should touch on all the important elements of your paper, but without an ounce of fat. Methods can usually be reduced to a sentence or less in an abstract, telling the reader the type of experiment or study being performed. It is usually easiest to write your abstract after the paper is completed although a good outline should also provide an excellent starting point.
The best way to get a “feel” for scientific abstracts is to read some. You can do this by walking down to the library and browsing through the current periodicals. Another option is to view abstracts online. Some computer databases of journal articles include the abstract. One that includes biology papers is Medline, which can be accessed here. Library staff can assist you if you need help using computer databases.