PSD what makes a logo good

These logos are well known and all have characteristics that define them from looking at the first logo I can tell that it is the WWF and that they work with animals so the image plays a big part of making the reader understand what the organisation  that the logo is for does.

The Nike logo is different from the  WWF logo as it has writing and the image is just a tick so some one that may have not heard of Nike and if they just saw the logo they would not understand what it is about unless they looked on the logo on some branded clothes.

The Mrozzia firefox logo  is harder to understand as the firstime I looked at it I did not know why the program was called Mrozzia firefox  now though I can see the orange fox but I still don't know  what the  blue bit is.

The top three things that make a logo suitable are:

1) Clear image
2) Keep a color shceme to make the logo presentable
3) If there is text make the font fit with the image