The main difference between the two is that the goal seek function allows the user to use the desired result of a formula to find the possible input value necessary to achieve that result. While in a data table, you can change values in some of the cells and come up with different answers to a problem.
You can create one-variable or two-variable data tables, depending on the number of variables and formulas that you want to test.
Use a one-variable data table if you want to see how different values of one variable in one or more formulas will change the results of those formulas.
Use a two-variable data table to see how different values of two variables in one formula will change the results of that formula.
Learn how to use data table and Goal Seek functions at Microsoft Support page.