Bias is the distortion of results by a variable. Some of the common biases that may impact the results of a research study are:

Sampling Bias

Sampling bias occurs when the sample studied in an experiment does not correctly represent the population the researcher wants to draw conclusions about.

Example: A psychologist wants to study the eating habits of a population of New Yorkers who are between eighteen and forty-five years old. Because she can’t study the entire group she’d have to take a sample. However, she can generalize her results to the whole population only if her sample is representative of the population. If this is not the case, her sample will reflect sampling bias.

Subject Bias

Research subjects’ expectations can affect and change the subjects’ behavior, resulting in subject bias. Such a bias can manifest itself in two ways:

Placebo effect

It is the effect on a subject receiving a fake drug or treatment. Placebo effects occur when subjects believe they are getting a real drug or treatment even though they are not. A single-blind experiment is an experiment in which the subjects don’t know whether they are receiving a real or fake drug or treatment. Single-blind experiments help to reduce placebo effects.

Social desirability bias 

It is the tendency of some research subjects to describe themselves in socially approved ways. It can affect self-report data or information people give about themselves in surveys.

Experimenter Bias

Experimenter bias occurs when researchers’ preferences or expectations influence the outcome of their research. In these cases, researchers see what they want to see rather than what is actually there. It includes Expectancy Bias, Personal Bias and Observer Bias.

A method called the double-blind procedure can help experimenters prevent this bias from occurring. In a double-blind procedure, neither the experimenter nor the subject knows which subjects come from the experimental group and which come from the control group.

Following is a list of various biases in psychology by category:
1) Cognitive bias: Confirmation bias, Negative bias, Gender bias, Anchoring bias, Memory bias, Overconfidence effect, Positive outcome bias, Optimism bias, Attentional bias.
2) Social bias: Actor-observer bias, hindsight bias, Egocentric bias, Notational bias, Outgroup homogeneity bias, Projection bias, Self-serving bias, Trait ascription bias, cultural bias, correspondence bias.
3) Research bias: Social desirability bias, Measurement bias, Experimental bias, Design bias, Quantitative research bias, Qualitative research bias, Selection bias, Systematic bias, Choice-supportive bias Confirmation bias, Congruence bias, Distinction bias, Information bias, Omission bias, Outcome bias, Status quo bias, Unit bias, Zero-risk bias, Subject bias.

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When the result of a research is distorted due to some factor we say it is biased. Research biases are of many types – the common ones are sample bias, subject bias (includes placebo effect and social desirability effect) and experimenter bias (includes expectancy bias and observer bias). There are various other kinds of biases as listed above. Some of the ways to control biases include simple-blind and double-blind experiments.

Sources:
http://www.psychwiki.com/wiki/What_are_the_types_of_bias%3F http://www.sparknotes.com/psychology/psych101/researchmethods/section3.rhtml