What is difference between upgrade and custom installation?

Sometimes when you begin to install a program, you will be asked if you want a "typical" or a "custom" installation. If you click on typical, the program will be installed with the components and features that the manufacturer has chosen. This may include separate additional features or programs that you may not be aware are being installed. If you choose a "custom" installation, you may pick which features and sub-features are installed. However, each installation is different and the "custom" choice may offer you options that you do not understand. If you don't understand the various program components that you are being offered, then you can usually just click on the cancel button and install the program with the "typical" features.