I don't understand, so windows 10 will be installed on to my computer even though I don't want it?
No of course not. They will tell when you can download it, but you don't have to. If you are a Windows 7 user it's best you don't download it straight away, till the computer nerds have worked out it will work and pass the verdict on it. Plus if it's no good then there will be a lot of egg on Microsoft's faces and the company will be in deep s*** .
But if the nerds love it, which I think they might well, you can try it out also and if you don't like it remove it and revert back to the old system.
There going to be plenty of new features on Windows 10, plus it will actually be smaller in size then Windows 7, so your computer might run better with it. Of course it's a question of personal choice, but you have only a limited time to download it for free and they mean free, after that they will hit you with a bill of over £100.
The only thing that might stop you downloading Windows 10 is that some older programs or specialist ones might not work with it. But if you are on Windows 8 or higher than they should.