what is the difference between back test & demo test?
is back test result believable?
A backtest is testing your system with some years or decades of historic price data. This is normally done by a computer. If a backtest result is useful or not depends on the system. In most cases a simple backtest is meaningless, you need more complex testing methods like WFA for determining the robustness of a trade system.
A demo test is not a test, it is just opening a demo account and entering some trades of your system. This does not test the system, it just makes yourself familiar with manually trading it.
that means backtest result of any EA isn’t actual performance?
Yes. For normal systems, a simple backtest is only a weak indicator of real trading performance. That’s why in some trading books, more realistic test methods are referred to as “forward tests”, although they in fact all use past price data.
Here’s a page from the Zorro manual with some info about system testing: Zorro Manual