With this fairly broad classification I set about working out exactly what a Focus Cloud should show. To me the name could only suggest one thing. It shows the area which currently is receiving the most Focus! [...] So a Focus Cloud should show not what has been the most popular tags overall, but what is the most popular tags at present. What is your current focus.
The post not only includes the theory behind these focus clouds but also has some PHP code to back it up (using the del.icio.us interface and tags as the foundation of the cloud's contents). The full code for the cloud can be grabbed here.