Hard work refers to work a lot without thinking whereas smart work refers to work less but with thinking and doing the same amount of work with good quality.
A person who does hard work may or may not get a good result. It depends on his way of doing work. For example if is trying to cut a tree with a blunt axe it will obviously take more time. If you are a student then you should do hard work along with choosing a right way. For example if you are reading a chapter and you are not understanding it and only cramming then it will be not beneficial to you have done hard work but you will not get good result. Therefore, you it is important to choose a right way to do work not only hard work will do it.
On the other hand is smart work which take less time but will make the quality of work better. Smart work not only saves your time but also makes your work beneficial for future. In the first example if the person used a sharp axe in place of blunt axe his work would be finished earlier. For a student smart work is to fix a time in a day for studying in place of studying whole day during exams.
Smart work is better than hard work in some cases but not in all cases. For example once a there were two people growing same crops in two different fields. One person is sowing seeds individually and the other scattered the seeds in the field together. First person watered the seeds individually and the other being over smart watered them using pipe. After one month the result was that the field of first person was better than the other. The first field was looking nice and the other field was untidy and looking as it was spoiled. Conclusion is that we should not be over smart. A person should use his brain along with hardworking then he will definitely be able to fulfill his work properly.