Holiday Inn usually features a wider selection of amenities, such as full-service on-site restaurants, perhaps large entertainment centers, things like that. The prices will usually reflect these extra services.
In a Holiday Inn Express, the rooms might look roughly the same as they do in the regular chain.. You'll have your bedding, bathroom, phone, probably a TV, probably Internet access, a coffeemaker, things like that. The differences will be in the extent of the establishment's services. An Express might have a basic continental breakfast instead of a full-service restaurant. It might have a pool, but probably won't have a large entertainment atrium. It might have a meeting room or two, or maybe a computer in the lobby.
In short, the differences largely come from the available amenities. I've stayed in both types of establishments many times over the years, and generally been satisfied with both.
Hope this helps!
49-state US road tripper. Most recent Holiday Inn stay July 2007, an Express in Saskatoon, Sask.