What is 'ecomonies of scale' and how does it differ from diminishing returns.

Economies of scale is when the price of producing a good goes down as a firm expands and produces a greater output. In other words, it's the economies of mass production. A firm produces so much of a good that the cost to produce the goods is less than if they were to have produced less at a time.
The law of diminishing returns states that as successive units of a variable resource (such as labor) is added to a fixed resource (such as capital or land), beyond some point, the extra (marginal) product of each additional unit of the variable resource will decline.
That is to say, as more of the variable resource is added, eventually the change that that resource adds will be smaller than the last.
For example, a firm has a constant amount of capital, but they hire some more employees. As more employees are added, the output will increase, but as more and more employees are added, the difference that one employee makes will not be as big as the one that was hired before him.