Work has a very specific definition in physics and it is actually a very usable definition in everyday life. You can have a look at some external sources for in depth explanations or otherwise just keep reading and you will probably want go to those after that .
Khan Academy video on work and energy
Wikipedia article on work
Now we are going to strip down the definition and still try to make this post short. To begin with, we will use a very simplified, relaxed definition of mechanical work:
"Mechanical work is the result of a force applied to something, providing a change results as the result of the force applied."
In other words, if you move something, your are doing some mechanical work. If you don't move it, no matter how much force you used (how much effort you made) no work was done. Actually if you move something, and the bring it back, lets say, just for the sake of it, no work has been done!! Or it was done and then undone if you want to see it that way.
Now... in order to do work, you need to spend energy. Actually you need to spend at least a little more energy than the work you get done. Also, you can spend a lot of energy and get little work done... or even none done.
Doing work requires spending energy. Spending energy does not imply work is being done.
And so we get to the point of this post:
If work is done, something has been changed, if that change is of some use to someone, it can be consider a finished job. For that time and effort we get, in exchange the promise of someone else's time and effort (read previous post ).
In the next post we see will how we can define effort, multiply it by time, and mathematically define this work.