The MD4, MD5 and SHA-1 algorithms are secure hash functions. They take a string input, and produce a fixed size number - 128 bits for MD4 and MD5; 160 bits for SHA-1. This number is a hash of the input - a small change in the input results in a substantial change in the output. The functions are thought to be secure, in the sense that it would require an enormous amount of computing power to find a string which hashes to a chosen value. In others words, there's no way to decrypt a secure hash. The uses of secure hashes include digital signatures and challenge hash authentication. This document is a good introduction to hashes
hex_md4("test hash") = "549089516e75bd13c41ff098fbb58d5e" hex_md5("message digest") = "f96b697d7cb7938d525a2f31aaf161d0" hex_sha1("160-bit hash") = "90d925d853c3d35cd54070bb75280fefad9de9e7"