The manipulated variable in an experiment is the independent variable; it is not affected by the experiment's other variables. HowStuffWorks explains that it is the variable the experimenter controls. When there are control and experimental groups, the manipulated variable is the treatment supplied to the experimental group and denied the control group.
When a scientist graphs the results of an experiment, he graphs the manipulated variable on the x-axis of the graph. One common manipulated variable is time. The scientist controls the time at which he makes the measurements. The y-axis represents the response or dependant variable. Temperature is a common response variable.
Because it is easy to confuse variables in the experiment, About.com offers the mnemonic DRY MIX to help students. It translates: dependant, response, y-axis; manipulated, independent, x-axis. Another way to help keep these values from being confused is through remembering that the independent variable is the "I do" variable.
In addition to manipulated variables and response variables, experiments often include controlled and extraneous variables. Controlled variables are those which the experimenter attempts to keep the same for both groups. Extraneous variables often change the outcome of the experiment in an accidental or unanticipated way. While the experimenter does not graph these variables, it is important for him to keep a record of them in his notes.