When doing psychological research, you will need to present your results in a concise manner that allows you to draw inferences based on observations. Oftentimes you will be looking at how altering one aspect of an experiment affects other parts, and when you interpret the results you will be looking for links between these phenomena. These links are known as correlation.
For example, say you do a study of anger in red-headed people. You design an experiment and test it, and while looking at your results you notice that the subjects who are out of work were the angriest. This is a correlation, and although it does not necessarily mean jobless people are angry or vice-versa, it does indicate a link between unemployment and aggression. If you graph correlation, with the two variables on the x and y axes, you can see how linked the phenomena are. If the phenomena seem very tied together (like humidity and hot weather), they have a high correlation. Low correlation would indicate that each phenomenon has little or no influence on the other (like humidity and earthquakes).
Central tendency is a term that describes trends in a set of data, and the measures of central tendency allow you to make generalizations about the set. The three basic measures of central tendency are mean, mode and median. They are used to describe different aspects about a set of data. For example take this set of test scores: 90, 78, 86, 90, 80.
The mean is the average of the set, which you calculate by first adding together all of the values in the set (for this you add all of the scores). Once you have a total, divide by the number of items in the set (there are 5 scores, so divide by 5). In this case the average score is 84.8.
Mode is simply the value that occurs most often in a set of data. In the example set of test scores, 90 occurs twice and all others appear only once. For this set of data, 90 is the mode.
Sometimes you will want to find the exact center, or median, of a set of data. First, order the data from smallest to largest (78, 80, 86, 90, 90). The value in the exact middle is our median, or 86 for this set. If there had been another value, the center of this data would fall between two numbers. In that instance, you will take the average of those two values. To illustrate, add a score of 88 to this set and then order it (78, 80, 86, 88, 90, 90). The median now falls between 86 and 88, the average of which is 87.