Question
What is the difference between floribuda roses and shrub roses?

Answer
Technically a floribunda is a hybrid tea that has been crossed with a polyantha. From this cross you get more flowers from the polyantha and a better shape from the hybrid tea. Floribundas usually make a smaller and more dense bush than a hybrid tea. and require a small amount of care to survive.
Modern shrubs used to be a catch all class when the breeder couldn't fit a hardy type rose into any of the other rose classifications. There are many forms, shapes, sizes and colours including those that only flower once and those that repeat. The Austins are a good example of a modern shrubs as they have been crossed with hybrid teas and the old garden roses. Most modern shrubs are usually hardier than hybrid teas or floribundas.
Now to make it even more complicated, the modern shrub classification has been broken down into landscape type roses. These are types like Knock Out rose which don't have perfect flower shapes but will bloom and survive without much care in places that most roses would not do well.  In the States, it is The American Rose Society that places a rose into the classification that is best for it.