Source Insight can compare two files and show differences. It can also compare two different directories.
The Tools > Compare File command compares two files to show differences within the files. It shows two files side by side and highlights the differences. This is a fancy version of the diff command. See: Compare Files. See: File Compare.
The Tools > Compare with Backup File command compares the current file with a backup version of the same file. See: Compare with Backup File.
The Tools > Directory Compare command compares two file directories to show what files are different between them. See: Directory Compare.