On a cold day, ice forms quickly on the surface of a pond. But it does not grow as a smooth, featureless covering. Instead, the water begins to freeze in many places independently, and the growing plates of ice join up in random fashion, leaving zig-zag boundaries between them. These irregular margins are an example of what physicists call "topological defects" - defects because they are places where the crystal structure of the ice is disrupted, and topological because an accurate description of them involves ideas of symmetry embodied in topology, the branch of mathematics that focuses on the study of continuous surfaces.
Current theories of particle physics likewise predict that a variety of topological defects would almost certainly have formed during the early evolution of the universe. Just as water turns to ice (a phase transition) when the temperature drops, so the interactions between elementary particles run through distinct phases as the typical energy of those particles falls with the expansion of the universe. When conditions favor the appearance of a new phase, it generally crops up in many places at the same time, and when separate regions of the new phase run into each other, topological defects are the result. The detection of such structures in the modern universe would provide precious information on events in the earliest instants after the Big Bang. Their absence, on the other hand, would force a major revision of current physical theories.
The aim of this set of Lectures is to introduce the reader to the subject of topological defects in cosmology. We begin with a review of the basics of defect formation and evolution, to get a grasp of the overall picture. We will see that defects are generically predicted to exist in most interesting models of high energy physics trying to describe the early universe. The basic elements of the standard cosmology, with its successes and shortcomings, are covered elsewhere in this volume, so we will not devote much space to them here. We will then focus on some specific topics. We will first treat conducting cosmic strings and one of their most important predictions for cosmology, namely, the existence of equilibrium configurations of string loops, dubbed vortons. We will then pass on to study some key signatures that a network of defects would produce on the cosmic microwave background (CMB) radiation, e.g., the CMB bispectrum of the temperature anisotropies from a simulated model of cosmic strings. Miscellaneous topics also reviewed below are, for example, the way in which these cosmic entities lead to large-scale structure formation and some astrophysical footprints left by the various defects, and we will discuss the possibility of isolating their effects by astrophysical observations. Also, we will briefly consider gravitational radiation from strings, as well as the relation of cosmic defects to the well-known defects formed in condensed-matter systems like liquid crystals, etc.
Many areas of modern research directly related to cosmic defects are not covered in these notes. The subject has grown so wide, so fast, that the best thing we can do is to refer the reader to some of the excellent recent literature already available. So, have a look, for example, to the report by Achucarro & Vachaspati  for a treatment of semilocal and electroweak strings (2) and to [Vachaspati, 2001] for a review of certain topological defects, like monopoles, domain walls and, again, electroweak strings, virtually not covered here. For conducting defects, cosmic strings in particular, see for example [Gangui & Peter, 1998] for a brief overview of many different astrophysical and cosmological phenomena, and the comprehensive colorful lecture notes by Carter  on the dynamics of branes with applications to conducting cosmic strings and vortons. If your are in cosmological structure formation, Durrer  presents a good review of modern developments on global topological defects and their relation to CMB anisotropies, while Magueijo & Brandenberger  give a set of imaginative lectures with an update on local string models of large-scale structure formation and also baryogenesis with cosmic defects.
If you ever wondered whether you could have a pocket device, the size of a cellular phone say, to produce "topological defects" on demand [Chuang, 1994], then the proceedings of the school held aux Houches on topological defects and non-equilibrium dynamics, edited by Bunkov & Godfrin , are for you; the ensemble of lectures in this volume give an exhaustive illustration of the interdisciplinary of topological defects and their relevance in various fields of physics, like low-temperature condensed-matter, liquid crystals, astrophysics and high-energy physics.
Finally, all of the above (and more) can be found in the concise review by Hindmarsh & Kibble , particularly concerned with the physics and cosmology of cosmic strings, and in the monograph by Vilenkin & Shellard  on cosmic strings and other topological defects.
2 Animations of semilocal and electroweak string formation and evolution can be found at http://www.nersc.gov/~borrill/ Back.