Glossary:compression
From Wildsong
File Compression
bzip2 bzip pkzip
lzw
Limpel-Zev-Welch was a great compression type until someone jumped up and said "I own it! Pay me!" and everyone had to either stop using it or pony up. Thus if you have old compressed [[Glossary:TIFF|TIFF] images, and you try to open them in ArcMap, you get a dialog box asking you to send more money to ESRI.
Workarounds: you can decompress the TIFF files outside of ArcMap and store them in an uncompressed format, or you can decompress them and convert them to another format.