Thus a dictionary is also compressed with your file. By using an adaptive algorithm the program constantly looks for more and more efficient ways to compress the data thus making your file size even smaller. The longer the file the more chances that a recurring pattern will be picked up by the compression software thus the more compression can be carried out. Word processing documents and databases generally have many more patterns than graphics files therefore are able to be reduced much more. When you unzip a file the computer reads the zipped file, as it comes across the pointers it goes to the dictionary and picks up a copy of the phrase and puts it back into the spot where the pointer was.
Some files contain their own forms of compression.
Graphics files for example use a lossy compression which uses a hard edged manner whereby they discard forever small bits of data such as a variation in a colour which won't be noticed, thus lossy zipped files can't be unzipped to an exact replica of their original state. A zip drive is an external or internal piece of hardware that can be used to compress your files onto a special disk. The disk can hold between 100-250 Mb of data. The disk is similar in shape to your 1.5Mb floppy but it is thicker and harder. It has a special magnetic coat and is protected by a hard plastic shell with a metal plate that is able to slide open to give the read and right heads on the disk access.