Stromboli is quite separate from the other Aeolian islands, to the north-east of the main group, and is as close to mainland Italy as it is to Sicily.
The island is best known for its dominant volcano - a volcano that is constantly active (typically 'small' eruptions take place several times an hour) and frequently has more substantial eruptions (it is one of the most active volcanoes to be found anywhere on earth).
For a long time the eruptions were reasonably predictable and tourists were able to visit the summit of the volcano, but in recent years this has changed slightly and since 2007 (when two new craters appeared on the volcano) the summit itself has been frequently closed to visitors for reasons of safety. The viewing platform at Sciara del Fuoco is only 400 metres away and also provides impressive views.
At all times there is limited access to the summit - a local guide is necessary (can be arranged in Stromboli). Note: it is also a 'proper' walk requiring sturdy shoes and a general level of fitness.
Remarkably perhaps there are two small villages on Stromboli, and a few hotels etc for those wishing to pass the night (eruptions are at their most impressive when seen at night). Stromboli is the larger of the two and has the facilities (and is where boats arrive) while Ginostra is much more remote.