There are a few El Hierro summaries out there, and various sources of information relating to the recent eruption, this post is meant as a short summary to summarise briefly what has happened from beginning to end!
Earthquakes during a period in July.
Earthquakes began in July with a clustering of many small <m 3.0 tremors beneath the island. These were located in a broadly oval shape beneath the island of El Hierro, with the majority situated between 10 and 16 km. These earthquakes mainly clustered in the pattern shown to the right. Since then speculation has occurred on whether an eruption would occur.
Before the eruptive episode began there was only minor inflation and little major increase in gas release to suggest a large movement of magma. An undersea eruption likely started before the 11th October and will have looked largely like the video posted below (with the formation of pillow lavas). This was indicated by the intiation of more shallow earthquakes off the coast. Since then a combination of gas bubbles and discolouration of the surface has confirmed peoples suspicions of an undersea eruption. A number of fish have also been seen floating near the surface, a result of the mixing volcanic gases and seawater causing the pH to rise.
Most recent news suggests that the activity so far is dying down, and is therefore unlikely to create a new island as dramatically as seen at somewhere like Surtsey, Iceland. It is important to note here that activity can change, although in this case, it seems unlikely.
As always you can visit the Eruptions Blog for more detailed information and
discussion, earthquake-report also has a good overview from beginning to end.