The standards required for a document intended to be viewed on a screen are different than those for a document intended to be printed.  This is especially true for graphics.

There are two things that influence how graphics turn out as they’re printed.

    1- The resolution
    2- The color space

Resolution refers to the number of pixels within an image, and their density.  Unfortunately, just because an image looks good on a screen doesn’t mean that it will necessarily look good as it is printed.  This is because there is a wide variation between standards for screen resolution and for print resolution.
The default resolution for many screen applications is 72 pixels per inch (ppi, sometimes referred to as dots per inch, or dpi).  But print resolution is 300 ppi…more than 4 times higher than screen resolution.  A 72 dpi image, then, will look great on-screen, but will often look washed out or pixelated when printed.
The closer you can get to 300 dpi, the better the print result.  The rule of thumb is to use the highest resolution possible for images; they can easily be down-sampled to 300 dpi if necessary, but they can’t be upscaled.

Color space refers to the system used to render color.  Most screens natively use an RGB (Red, Green, Blue) color space.  When printed at a professional printer, however, that must be changed to a CMYK (Cyan, Magenta, Yellow, and Black, or “4-color”) color space.  The CMYK values define the colors that will be mixed as pigments on the page.

Usually, the consequences of this change are imperceptible.  For instance, photographs tend to translate very well.  Here are two sets of photographs, the first of each set being rendered in RGB and the second in CMYK:

There are some consequences of a CMYK color space, though, particularly in regard to more vibrant colors.  It is possible to create colors in RGB that can’t be replicated in CMYK.  They are said to be “out of the CMYK color gamut.”  When using these colors, what one sees on the screen can’t be perfectly recreated in ink.  For instance, here is an RGB image as it converts to CMYK:
    
Or here, where only the bright green color seems to be affected, but the pink is not:
    
Professional print designers typically work within a CMYK color space for this reason…so that they know with more certainty what a print result will be.  Unfortunately, Word, and also many PDF creation programs, only work within an RGB color space, so most users will have to simply be aware that certain colors, especially more vibrant colors, may end up appearing somewhat muted in their printed copies.