  This text tries to give an elementary introduction to the mathematical
properties of infinite sets. The aim is to keep the approach as simple as
possible. Advanced knowledge of mathematics is not necessary for a proper
understanding, and there is (almost) no use of formulas. At the same time, it
is tried to keep the reasoning rigorous and transparent. Major arguments are
explained in great detail, while it is explicitly stated if something is not
further explained.
  However, also a fully guided trail can become difficult at times. Persistence
is necessary to follow. No reader should be discouraged when he or she cannot
understand everything on first readings - infinity is not a piece of cake.
  The reasoning presented here is mainly due to the famous German mathematician
Georg Cantor (1845 - 1918). Despite the resistance from the mathematical
community at that time, he opened the door to infinity almost on his own,
dramatically changing the foundations of mathematics.
  The following notions and topics are treated: potential und actual infinity,
cardinality and countability, Hilbert's hotel, Cantor's diagonal arguments and
the highly astonishing answer to the question of the validity of Cantor's
continuum hypothesis.
