A prime example is ISO images for operating systems like Linux and Windows. A couple of popular hash algorithms are MD5 and SHA-1 and you will sometimes see these values listed on website download pages. If even one byte in the file changes, the value given when the check is run again will be different.
Every file has unique data contained within it, and when you apply a certain algorithm called a “cryptographic hash function” to it, a string value is returned which is only valid for that file in its current state. Or you can verify a file’s integrity by checking its hash value. One of the ways you can identify whether a file has been changed from its original state is to check its digital signature. This could either be by the site you are downloading from, corruption due to errors in the download process, an individual who has uploaded the file for you, or possibly the most dangerous, the file has been infected by malicious software. When you download a file from the internet, quite often you cannot be 100% guaranteed that the file has not been changed in some way from the original.
