Type I Error
A Type I Error is also known as a False Positive or Alpha Error. This happens when you reject the Null Hypothesis even if it is true. The Null Hypothesis is simply a statement that is the opposite of your hypothesis. For example, you think that boys are better in arithmetic than girls. Your null hypothesis would be: "Boys are not better than girls in arithmetic."
You will make a Type I Error if you conclude that boys are better than girls in arithmetic when in reality, there is no difference in how boys and girls perform. In this case, you should accept the null hypothesis since there is no real difference between the two groups when it comes to arithmetic ability. If you reject the null hypothesis and say that one group is better, then you are making a Type I Error.
See also: Type II Error