there are different varieties of creationism. some of them are consistent with the theory of evolution by natural selection (on this account god made the world with the natural laws and evolution by natural selection was the process by which he made the diversity of life that we see before ut). other varieties of creationism directly contradict some evolutionary claims, however.

e.g., the 'young earth' people who maintain the earth is much younger than what scientists have estimated - in virtue of some passage in the bible.

there are people who are literalists about Genesis too. They think that God literally made the world in 6 days and that when he made the animals and plants he made each 'according to its kind' (which they take to mean species). this is an outright denial of the common ancestor claim. it denies that natural selection results in speciation. denies the cladestine classification system in biology (where different species are given their place in the evolutionary tree according to how long ago they speciated / diverged from other lineages).

trouble with denying this... is that we can see evidence of evolutionary processes. moths, for example... start off with a population of black ones and white ones... black ones get eaten more 'cause white ones are camoflagued on white bark... eventually... the population consists mostly of white moths. that is a process of evolution by natural selection. you have natural variation in a trait (colour) that is heritable (passed on). you have competition (in this case to avoid predators) and the result is... differential reproduction (the white ones produce more offspring on average) and over time... the white trait will become far more prevalent in the population. then all of a sudden along comes the industrial revolution and the rise of factories. the trees are covered with black soot. eventually you get the shift back in moths colouration.

i don't think creationists mean to deny this - do they?

it is more some of the specific claims that are made within evolutionary theory (e.g., that natural selection can result in different species rather than merely changing traits within a species or that natural selection isn't able to explain such obvious adaptations as the eye or the feather).