Evolution is the leading theory that proposes how life has come to be as united and diverse as it is now. While evolution certainly has gaps like any other scientific theory, it is supported by an overwhelming amount of data, including the fossil record, biogeography, DNA sequencing, homology, and much more.
Intelligent design, meanwhile, is inherently religious in that it postulates that life was created by an "intelligent agent" based on the idea that life is simply too complex.
As seen in the 2005 case of Kitzmiller v. Dover Area School District, intelligent design is not a legitimate theory. Previous administrations have come out in support of this idea, which compromises the quality of science education in the United States.