why are downloads sometimes tagged md5, sha1 and other hash indicators? | I've seen this all over the place: 



> 
> Download here! SHA1 =
>  8e1ed2ce9e7e473d38a9dc7824a384a9ac34d7d0
> 
> 
> 


What does it mean? How does a hash come into play as far as downloads and... What use can I make of it? Is this a legacy item where you used to have to verify some checksum after you downloaded the whole file?