Modman lets you install/uninstall magento extensions without the use of Magento Connect.
Why would you use modman?
Say you have an extension that doesn't use Magento Connect--where you upload it's contents manually. When you want to uninstall that extension, you have to tediously/manually search the installation files/folders and remove them by hand.
With modman, this is done for you perfectly--no more tedious labor.
Let's first install modman.
1. Enable symlink support in magento: System > Configuration > Advanced > Developer , Under "Template Settings" > "Allow Symlinks" select "Yes"
2. Download and run the installer.