Forensic Biology is the analysis of body fluids, stains and other bodily materials to help solve a crime.  Typically, this involves the positive identification of blood, semen, or saliva and further genetic testing (DNA) to determine who the material may have originated from, typically the alleged victim, suspect or other involved party.

Many times an alternate light source, such as a laser is used to find stains on articles such as bed sheets or clothing. A piece of the stain is cut out and tested for identification. The most common types of tests performed are those for semen and blood, although they may also do tests for other body fluids like urine or saliva. A forensic scientist must know how to perform the test,and how to interpret the results. Some tests are definitive, for example if a stain is in fact blood. Others can only provide a “likely” answer, like in the case of saliva, where there is no absolute conclusion. A positive analysis for this type of test means only that a substance is indicated (probable).
A Forensic Biologist must use care not to overstate their conclusions.

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Once a stain is identified, the scientist is often asked perform DNA analysis to determine who its possible origin. The resulting profile is compared to the DNA profile of persons of interest, typically a victim and suspect. If the profiles “match” and the next step is to figure out what that match means. How many other people could be expected to have the same profile? Depending on amount of detail discovered in the DNA profile, this could range from a lot of people, to one in several billion, or essentially only a single person. With this type of strong match, DNA analysis if very similar to a fingerprint, and the analyst is able to testify in court that the substance originated from a specific individual.

 
For more information about forensic biology and other forensic disciplines, I find the best resource to be ForensicScienceHQ.com .

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