Considerations when designing a logo.
Normally you need to be able to reduce the colors in a logo. You need to consider how it will look when it's dropped to "low color" and to black & white.
As per the sample above.
1) Full color | This is for general usage.
2) Low Color | Normally kept at 4 colors in the given sample white is one of the colors. This is used for print when the number of colors is limited.
3) Black & White | Used when only black ink is available. It's best to avoid grey scales if you can.
4) Black & White Small | Used when the logo will scaled down a lot and details will be lost.
Transparency is also a consideration. In some circumstances you will want what's under the logo to show through in something other then the "box" shape of the image.
Vector vs Bitmap, The above samples source files are all vector. Because of this they can all be re-scaled to any size without quality loss. I.E you won't see blocky pixels, or blurry spots. To have bitmap images scale well you have to use very very large images.