Intensity is a measure of an earthquake determined from the observed effects, especially damage.
For a given earthquake, intensity normally decreases with distance from the epicentre.
The observations can then be compiled to make macroseismic maps showing lines of equal intensity.
Since earthquake intensity can be related to what people feel or experience during the shaking from an earthquake it is possible to use people as "intensity sensors" and collect earthquake felt reports for significant earthquakes.
Scientists can make use of these felt report surveys to compare the size of recent earthquakes to ones which occurred before modern scientific measurements (from about 1970 in the UK), but for which contemporary written reports are available. In this way earthquake records in the UK can be extrapolated back to the 14th century.
Scientist can now predict the likely intensity of an earthquake's effects as soon as it happens and produce 'shake maps' showing how the shaking will vary from place to place. Maybe it is time more emphasis was put on these intensity values in communicating earthquake size to the general public?
Magnitude is a number that indicates how much energy an earthquake has released.