Which version do you want to download?
Your system is MacOS X.7 (Lion) or more?
Choose the more modern 64-bits version…
Your system is MacOS X.5 (Leopard) or X.6 (Snow Leopard), or your computer has an old PPC processor?
Choose the 32-bit version…
Both versions have the same features, and are compatible too with all recent systems
…But each one is optimized for your current system ;-)