programs like winzip - what do they do to make the file smaller?
is there any trade off?

thanks

Compression applies an algorithm to the data in order to remove as much redundancy as possible. The efficiency and effectiveness of a compression scheme is measured by its compression ratio, the ratio of the size of uncompressed data to compressed data. A compression ratio of 2:1 (which is relatively common) means that the compressed data is half the size of the original data.

Some places where a correct selection of a compression scheme can make a key improvement in a project:
In a VoIP implementation, routers, switches, proxy servers, storage area networks, desktop computers, ...

The trade-off: More burden on the CPU as more compression/decompression is being performed.

For further reference:
http://computer.howstuffworks.com/file-compression.htm

http://en.wikipedia.org/wiki/File_compression

http://www.cisco.com/en/US/tech/tk713/tk802/technologies_q_and_a_item09186a008019be75.shtml

thanks alot, i can sleep again now