Download 'Windows installer' if you wish to have a wizard which helps you to install the program on your computer, you should have administrative privileges on it.
'Binary package' contains the executable files of the program TTCalc and the rest which is needed such as a help etc., you can download it if you do not like any installers.
TTCalc also appears in a portable version, in this version the program can perform calculations only with one kind of precision and is much smaller in size, you can easily put it into your pendrive and take it with you when you're travelling.
If you'd like to see how the program is written or want something to add to the program download 'Source code'. The package also contains the TTMath bignum library. You can also browse the source code online.