A checksum calculator is a tool for producing and then checking checksums. A checksum is a relatively short number that corresponds to a longer string of data, effectively a form of shorthand. The main use of a checksum is to give an easy way to check that data has not been corrupted. If the data itself has changed, the checksum will also change.
The main use of a checksum calculator is to protect against accidental damage. This could happen if data is corrupted in transit, which can happen easily, given the way data is broken apart and reassembled while passing through networks and the Internet. A checksum doesn't usually offer much protection against malicious damage, as the checksum process can usually be run in reverse so that the content is changed but the checksum remains the same.
There are many different techniques that can be used by a checksum calculator, with varying degrees of sophistication. Some work extremely simply and are very roughly equivalent to counting the number of characters in each chunk of data. Naturally, these are much more vulnerable to failing to detect simple errors such as two pieces of data accidentally switching place. More complex techniques involve taking account both of each chunk of data's individual characteristics and its position within the entire string of data.
Sometimes, a checksum calculator will work with very small pieces of data. These are sometimes also known as check digits. One example would be with Social Security numbers, ISBNs from books, or the numbers on barcodes. These contain a single check digit at the end that corresponds to the rest of the number. Most electronic tills or computer systems will compare the number and the check digit when they are typed in. If they do not match up correctly, it will be clear that a typing error has been made.
One common checksum calculator is md5sum. This works with 128-bit hashes using the MD5 format, which is commonly used for two main purposes. One is for security to check that a file has not been tampered with and made to carry a virus. Another is for the ISO format that is used to carry the entire contents of a CD or DVD in a single file for easier transferring. Using the MD5 checksum helps avoid the problem of a single error making the entire file, and thus the entire disc, unusable.