1.Correlation definition states that there is some form of a connection between two or more studies that have been researched in which it produces either a negative or positive outcome on any given research provided but ultimately depends on the specific research that is being acted on. For an example the positive effect is that when two things are attracted through a strong connection which provides the positive effect while the negative effect is provided through a connection that is less connected and falls farther apart from each other. You also must compare such connections for them to have the results you are looking for so they can provide a much more accurate representation of data.
2.First you have to know that there are at least 4 different tests that are available to compare differences and means. There are the independent samples T-Test. You would use this test when the samples are independent from each other and you want to compare them. Next is the one sample T-Test. This one you would use if you only have one group that is going to be tested 2 different times. Meaning that you would have 2 separate measurements on the same item or person. 3rd is the paired samples T-Test. You would use this one if there is one group that is being tested at different times. Lastly the ANOVA or one way analysis of variance. This one you would use when you have more than two levels of independent variables.