What is the difference between a mashup and a Portal? Don't they both just collect disparate data and present it in one central area?

Mashups do not require an interface, while portals are extremely UI-bound. Also, there are many parts to a portal; the portal server and the portal UI are the two major ones. In the Java world, the UIs are JSR-168 compliant portlets. Presto produces Mashlets that are JSR-168 complaint and thus can work inside any portal.