A Z-test is a statistical method used to determine if there is a significant difference between sample and population means or between two sample means. It is applicable when the sample size is large (typically over 30) and the population variance is known. The test uses the standard normal distribution to calculate the Z-score, which indicates how many standard deviations an element is from the mean. Z-tests can be one-tailed or two-tailed, depending on the hypothesis being tested. A one-tailed test checks for a difference in a specific direction, while a two-tailed test checks for any difference. The results help researchers make informed decisions based on their data.