The most basic difference, as all answers here stated, is that a jig is a type of tool used to hold and support the workpiece but in addition to this, Jig also control the location or motion of tool. On the other hand, a fixture is a support or work holding device used to hold work in place. It never guide the tool.
That’s the difference. Jigs guide the tools (in simple words, jigs instruct tools that where to go) but fixture never guide the tool. Below images may clear this further.
Figure 1. Shows Jigs.
In figure 1 (above), U shaped body is Jig because as we can see, it has holes in upper portion which will guide the boring bar so that it can do boring operation in workpiece.
Figure 2. Shows Fixture.
Figure 2 is nearly same as figure 1 but one basic difference is in upper part of U shaped body. You can see there is no holes in this image and so this U shaped body can not guide the boring bar. Here, it will just hold and will support the workpiece.
So, in figure 2, U shaped body is fixture while in figure 1, U shaped body is jig.
One another example in figure 3.
Left is jig as it has hole to guide tool, while right one is fixture as it is just holding the workpiece.
Now, some more differences are:
Jigs are light in weight as compared with the fixtures.
The cost of jigs is higher when it is compared with the fixtures.
Jigs are not fitted to the machine table but fixtures are fitted.
References: Images from Google
Difference Between Jig and Fixture
Difference Between Jigs and Fixtures
What is the difference between Jigs and Fixtures