||Concurrent = Floating, meaning a license manager is running somewhere on a network server w/a hardware key (if a Windows box) containing a certain number of features or licenses for the various desktop products
If you have concurrent licensing, you can install the software on many machines. Then you set up through the install where the license manager is running (name of server).
Last license used up w/trigger a message to other users that there are no avail. licenses when they try to start an application (ArcMap, ArcCatalog) or turn on an extension.
Single-use is tied to one machine and you can use various methods of registering the product (online, fax, etc.) before using it, but does not require a hardware key or license manager install.