iLok is a separate software that manages authorizations. Having an iLok USB allows you to carry your software licenses on a physical drive between a studio, your home and any other location you may be using iZotope software.
iLok is an optional authorization method with all iZotope software, you can have a computer authorized as well as an iLok USB if you wish.
Exponential Audio products all require iLok authorization to work but do not require an iLok USB. You can manage your iLok Authorizations via the iLok License Manager. To read more about authorizing Exponential Audio products, on the Exponential Audio authorization article.
You can get more information from the iLok homepage: https://www.ilok.com/#!home