Topological defects are stable configurations of matter formed at phase transitions in the very early universe. These configurations are in the original, symmetric or old phase, but nevertheless they persist after a phase transition to the asymmetric or new phase is completed. The type of defect formed is determined by the symmetry properties of the matter and the nature of the phase transition. These are two-dimensional objects that form when a discrete symmetry is broken at a phase transition.
Domain walls have some rather peculiar properties.
For example, the gravitational field of a domain wall is repulsive rather than attractive. Domain walls are associated with models in which there is more than one separated mimimum. Strings can be associated with grand unified particle physics models, or they can form at the electroweak scale. They are very thin and may stretch across the visible universe. A typical GUT string has a thickness that is less then a trillion times smaller that the radius of an Hydrogen atom.