Hard links

Hey, I was just going through the ioncube manual (thinking of making something like that, but much cheaper so everyone can afford it....maybe $20?) and came accross this: 3.5.1 Copying with Hard Links [--use-hard-links] The Encoder will normally perform a file copy into the target for non-encoded files. This is fast but performance can be improved and disc space saved by using the --use-hard-links option to copy by using hard links. This feature is only available with Unix encoders and only if the source and target files are on the same filesystem. My (simple) question is: What is "hard links"? never really heard of it and googles no help either. Am I missing something? Thanks, -Ryan