Checksums provide a simple way to compute the integrity of data files before and after file transfer or in backups. A checksum is like a unique fingerprint of a file that can be used to verify whether two files are identical.
Each time you run a checksum a number string is created for each file. Even if one byte of data has been altered or corrupted that string will change. If the checksums before and after copying or backing up a data file match, then you can be sure that the data have not altered during this process.
A free software tool for computing MD5 checksums is MD5summer for windows. This tool computes checksums according to the MD5 checksum algorithm. No matter how large the data file, it always has a short 128bit MD5 checksum value that can be easily stored and shared, and compared against the checksum value of another file.