What is the difference between these derailleurs. I want to change my derailleur and don’t know which one is better.
If you want to know the exact differences, Shimano does provide very detailed specs on their Manuals & Technical Documents site. (Click 'Technical Documents' on the left hand side to get to line up, specs and compatibility pages.)
Specification are ordered by use (MTB, road, ...) and component. If you go to the MTB rear derailleur page you can pick derailleurs to compare.
In increasing quality (and expense) is Altus, Acera Alivio.
In real world conditions, there is not a lot of difference between each set. Experienced riders will probably notice that Alivio is better than Altus, but other factors such as shifters, cables and drive train wear make a larger difference
General advice I give to people is buy the right component - i.e. that matches the number of speeds and cage length - that's on the largest discount but still in your budget.