Online Ambassadors are experienced editors on the language Wikipedia where your program is in operation who are able to assist students with any questions they have through online support channels — on-wiki, on IRC, in a Facebook group, etc. They do not need to be geographically located near your students, since the support is all online.
In contrast, Campus Ambassadors aren't necessarily Wikipedia editors — although if they are, that's great — but they are volunteers who can work with instructors and students in person. Campus Ambassadors are often existing staff members within the school system (such as librarians, teaching assistance, or teaching and learning center staff) who are available to help teach students editing basics, such as how to create user accounts, add references, etc. They can often help instructors with assignment planning as well.
Some smaller programs collapse the distinction between Campus and Online Ambassadors and just have Wikipedia Ambassadors who serve both roles simultaneously.