Typing "Winrar recovery rate" into Google delivered a bunch of hits on password-cracking software for rar files. Rar, like Zip, is a data compression technology that also allows for password protection. My guess is that the encryption strength is pretty good, because all those programs indicated on Google touted their ability to do a brute-force attack quickly. In other words, try every possible combination of not only dictionary words but also random letters and hope for the best.
RAR has the ability to build-in a certain amount of headroom that can be used to recover the contents of the archive if it's partially corrupted. Say you have a 20MB file that gets RAR'd to 10MB. The person who RAR'd the file will have ticked the "put recovery record" option and then chosen "3%" as the value. The resultant RAR file will then be 10.03MB (10MB + 3%) so that if the file was corrupted along the way to you then WinRAR would still be able to decompress the file WITHOUT error as long as no more than 3% was corrupted.