It is a pleasure to acknowledge José M. Ibáñez and José M. Martí
for valuable suggestions and a careful reading of the manuscript.
The many colleagues who kindly helped me to update the previous
version of the review by providing relevant information are
warmly acknowledged. I am particularly grateful to Harald
Dimmelmeier, Konstantinos Kifonidis, Shin Koide, José V. Romero,
Masaru Shibata, and Wai-Mo Suen for providing some of the figures
and animations contained in the article. This research has been
supported in part by the Spanish Ministerio de Ciencia y
Tecnología (AYA2001-3490-C02-C01), and by a Marie Curie
fellowship from the European Union (HPMF-CT-2001-01172). The
author has made use of NASA's Astrophysics Data System Abstract
Service, which is gratefully acknowledged.