A Z-test is a statistical method used to determine if there is a significant difference between the means of two groups or between a sample mean and a known population mean. It is applicable when the sample size is large (typically over 30) and the population variance is known. The test calculates a Z-score, which indicates how many standard deviations an element is from the mean. To perform a Z-test, you first formulate a null hypothesis, which states that there is no effect or difference. Then, you calculate the Z-score using the sample data and compare it to a critical value from the Z-table to decide whether to reject the null hypothesis.