Bipmapped files are made of tiny dots. They can be made smaller, but become more and more jagged as they're made larger. Vector graphics are lines or fills enclosed within lines. They may be scaled either larger or smaller without losing sharpness.
Note: When using spot or named colors in a graphic that will be part of a larger printed publication, the color names must EXACTLY match the names used in the publication or extra printing plates will be made for each difference. This raises the cost unnecessarily and will delay a print job.
There are more file types than the following, but these are the most common.
JPEG
Bitmap. This is a lossy form of compression. High quality JPEGs will look
well printed, but the file size will be quite large and your friends will
be unhappy receiving them attached to emails. Low quality JPEGs have had a
lot of their pixel information thrown away or "lost." They are suitable
for emailing and viewing on a computer screen. Keep in mind that once a file
is saved at a low resolution it can never be returned to its original clarity.
Preserve your originals and save your lower resolution files with a different
name.
GIF
Bitmap. These files are low resolution and are limited to 256 colors which
makes them suitable for logos, icons, and graphics with solid colors. They
are not suitable for photos or printed materials. GIF files use lossless compression—no
information is thrown away in the compression process.
TIFF
Usually bitmap. These files are uncompressed, stable, and are preferred for
photos and graphics going to a commercial printer. They may be compressed
with WIN_ZIP or Stuffit without compromising their integrity.
EPS — encapsulated postscript
These files can be either bitmap or vector. If a bitmap, then they can be
resized smaller, but will become jagged when made larger. Vector files can
be used at any size. Warning: When a font is used in an eps graphic, the person
viewing or using that file MUST have the same font on their computer or another
font will be substituted — usually an ugly one. You can get around this
by converting anything in a font to "paths." However this makes
it uneditable, so keep your original. High resolution eps files are good for
importing into projects going to a commercial printer.
PDF — portable document format
Adobe has made this the industry standard for saving files containing text
and graphics so they will look the same no matter which operating system is
used or what fonts are installed. For this to work, you must embed the fonts
in the file. Set it to embed all fonts at 99%. (Using 100% sometimes causes
problems—don't know why, it just does.) The fonts will scale up and
down, but documents with graphics can only be made smaller.