The theory of intelligent design (ID) holds that certain features of the universe and of living things are best explained by an intelligent cause rather than an undirected process such as natural selection. ID is thus a scientific disagreement with the core claim of evolutionary theory that the apparent design of living systems is an illusion.
In a broader sense, Intelligent Design is simply the science of design detection -- how to recognize patterns arranged by an intelligent cause for a purpose. Design detection is used in a number of scientific fields, including anthropology, forensic sciences that seek to explain the cause of events such as a death or fire, cryptanalysis and the search for extraterrestrial intelligence (SETI). An inference that certain biological information may be the product of an intelligent cause can be tested or evaluated in the same manner as scientists daily test for design in other sciences.
ID is controversial because of the implications of its evidence, rather than the significant weight of its evidence. ID proponents believe science should be conducted objectively, without regard to the implications of its findings. This is particularly necessary in origins science because of its historical (and thus very subjective) nature, and because it is a science that unavoidably impacts religion.
Positive evidence of design in living systems consists of the semantic, meaningful or functional nature of biological information, the lack of any known law that can explain the sequence of symbols that carry the "messages," and statistical and experimental evidence that tends to rule out chance as a plausible explanation. Other evidence challenges the adequacy of natural or material causes to explain both the origin and diversity of life.