Defects are small impurities in otherwise perfect crystals. When describing crystalline solids we usually say that they exhibit a periodic crystal structure (a Bravais lattice), i.e. the cells (atoms or molecules) are arranged in a repeating pattern, which is symmetrical to discrete translations (determined by the unit cell's size). However, in nature we often see that the arrangement isn't perfect, and that some cells are misplaced (or missing) - this can happen at a single point in the lattice, or in large areas such as lines and even planes.
Defects can be grouped into two basic groups, according to their geometry:
Of course, many other defects exist, some of which are merely combinations of defects mentioned above.
The problems and disadvantages of working with defects are pretty clear: They're hard to describe accurately - since defects break the perfect periodic nature of the crystal, they also break its periodic potentials. This means adjustments must be made around the defect site when it's a single point, and when it's a plane or line defect, whole planes of atoms must be taken into consideration and re-evaluted. This is very difficult to do by hand (even approximately), but also expensive when done by a computer. The additional time needed to write the program, as well as to run it can be overwhelming.
However, as explained above, defects are commonplace when studying real-world crystals, and for that alone it is important to consider and explore them. But are there also advantages to defects? Can they be used for a practical reason?
The short answer is yes, but in a very limited way. The most popular usage for defects in using an NV defect for quantum computing1. Its robustness makes it a perfect candidate - its quantum state is easy to initialize, manipulate and measure2.