The Long Answer:
There are two fundamentally different possible causes for how humans have come to exist: blind natural processes (chance-law) or purposeful intelligent design. The two mechanisms are not wholly mutually exclusive over time, for some entities in the natural world may have come to their present forms due to some combination of chance-law and design. However, the two views stand in stark contrast to one-another as fundamentally different mechanisms for human origins. Some aspects of biology changes may very well be the result of Darwinan evolution (i.e. the mutation-natural selection mechanism), but some aspects of life may be due to design. Thus, in general, an organism life could be a combination of both forces at work on a species.
In particular, many proponents of intelligent design believe that microevolution is a strong force shaping life, but question if many macroevolutionary changes can be explained through the Darwinian mechanism. The term "evolution" simply means "change through time," but there are two types of evolution: macroevolution and microevolution. Microevolution is "slight, short-term evolutionary changes within species." (Futuyma, D., Evolutionary Biology, glossary, 1998) For example, within humans, there are different eye colors, hair colors, and skin colors. These are the result of microevolution. Macroevolution is "the origin and diversification of higher taxa" (Futuyma, D., Evolutionary Biology, pg. 447, 1998) or, "evolutionary change on a grand scale, encompassing [among other things] the origin of novel designs…" (Campbell, N. A., Reece, J. B., Mitchell, L. G., Biology 4th ed., pg. G-13, 1999). There is thus a fundamental difference in kind between microevolution and macroevolution.
What intelligent design does reject that the notion that mutation and natural selection (combined with any other natural mechanism) is sufficient to produce or explain all aspects of life. Rather than stifling scientific progress, intelligent design frees scientific explanations to being bound to purely naturalistic causes, which in many cases are proving impotent. In recognizing that intelligent design is a sufficient and acceptable cause for the origin of biological complexity, intelligent design adds a new valid dimension to potential explanatory causes for life.
It should be noted that although ID and evolution are compatible on the general level (i.e. life could be the result of both evolution AND Intelligent Design), on the specific case-by-case level, intelligent design is NOT compatible by evolution. A given nucleotide sequence either is the product of design or natural selection. That which is the product of design cannot be the product of natural selection, and visa versa. Thus, on the specific level, ID is not compatible with evolution. As William Dembski said during a talk at UCSD in 2001, "as far as the Darwinian mechanism goes, blessings to it." Dembski thinks its blessings stop at a certain point, and the information content at that point is so high, Dembski thinks it points to design.
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