Evolution
I wrote in a previous post about how many creationists dismiss the theory of evolution because they don't understand what a scientific theory means. Many of the other creationist criticisms that I have seen or heard seem to be based on a misunderstanding of the theory. (It's easier to knock down a straw man). Just because I felt like it, here is my explanation of the current theory of evolution in a nutshell.
Individual members of a species end up having a fewer or greater number of descendents depending on how good they are at (a) surviving and (b) fucking. Because every member of a species is different, over time the average characteristics of a population will shift in favour of traits that help them survive and fuck.
What those traits are depends on the current environment. The current environment includes both the physical environment (climate, geography, water and food supply) and other species (both higher and lower on the food chain or in competition).
What usually happens is that two (or more) groups of the same species end up in slightly different environments. What makes for the best survival and most fucking in one environment might be different from in the other environment. So, gradually (over thousands or millions of years), the traits of each group change over a period of time, until members of the two groups no longer recognise each other as the same species.
It can sometimes happen a bit faster too. Because the entire planet is an interlinked ecosystem, changes in one species affect others, rippling throughout the web of life. Changes in the environment (natural distasters, climate change, arrival of predators, dearth of prey) change the playing field. This can sometimes cause large number of individuals that were previously doing pretty well to suddenly cark it, while only those with some particular characteristic survive, resulting in a quite sudden change of the average characteristics of the population.
Although often called 'Darwin's theory of evoluation', Darwin himself didn't actually know _why _there was variation between individuals in a species, he only knew that there was, and worked out through careful observations of lots of species (but especially birds on the Galapagos islands) that this was what allowed the average characteristics of a population to shift over time. The modern theory includes the sudden changes (punctuated equilibrium) and explains (due to gene theory) how the individual variation happens - mutation, recombination and gene duplication, as well as interactions between different genes determining how the genes are exhibited.
Evolution does not say that man is "descended from" apes. Or monkeys. It says that both apes and humans have a common ancestor. We split into two groups that changed in different ways, probably because we were living in different immediate environments, making different characteristics more useful for surviving and fucking. The small cumulative changes can end up very large given enough time. Apes are not would-be humans that didn't "evolve far enough".
Evolution has no goal. There is no concept of a species "evolving towards" some final state, or devolving away from it. And specifically, humans are not "the goal", with apes representing a "lesser evolved" state. There is no such thing as a more or lesser evolved state. Evolution only appears to have cumulated in modern humans because we are here now, and we can't see into the future. It is equally true that an ape, or a fire ant, or an orange roughy are the 'final product' of these billions of years of evoluion.
Evolution creates new species. Some creationists will concede that micro-evolution is possible, with bacteria becoming resistant to antibiotics and all that. (Some actually deny it, despite current observable evidence). But they argue there's no evidence for macro-evolution, their term for when a some members of a species evolves so far away from others that they become a new species. Actually, there is heaps of evidence for it. DNA evidence for one, fossil evidence for another, but it has actually been observed several times just in the last few decades, with fruit flies and several species of fish.
Evolution does not violate the second law of thermodynamics. The second law generally says that entropy always increases, that is, things become more disordered over time until equilibrium is reached. Evolution (so far) seems to have created things that are more complex over time. The misconception is because the second law only applies to an isolated system, which earth is most decidedly not. Earth is in fact a complex self-organising system on the edge of chaos - where spectacular complexity can and (usually) does arise from very simple rules and building blocks (I might write more about this in future). So long as overall entropy in the universe increases, local areas of a system are free to have decreasing entropy.
Evolution is not random. Natural selection is not random. Creationists say 'it was either god or it was pure chance and something this complex can't have been just chance'. This really misinterprets the whole situation. It's not like you throw a whole bunch of amoebas in a pot and shake it, and then the first time you look you see a mess of amoebas, and the second time, just by "pure chance", out pops a fully formed human. It's just not like that. Rolling dice is random. Always picking the higher number on two rolled dice is not random. Evolution is pretty non-random.
Evolution does not explain abiogenesis. Darwin called his book The Origin of Species, not the origin of life. Evolution explains how, given living cells and time, you can account for the vast number of plant and animal species we observe. It does not explain how you get from non-living chemicals to living cells. That's the domain of abiogenesis. Criticising the theory of evolution for not explaining abiogenesis is like criticising the theory of gravity for not explaining why magnets are attracted to each other.
On a side note, many creationists seem to think that scientists view theories the way conservative christians view the bible. They believe in biblical inerrancy, so if any error can be found in the bible, it means that the bible isn't inerrant, and so can't have been written by god, and they basically have to throw out the whole thing. They force themselves to choose between either ignoring any contradictory new information gained since the bible was written, or accepting that the bible isn't inerrant. This essentially freezes scientific knowledge at two thousand years ago.
So they jump on tiny little things related to evolution (for instance): Ha! They said that Neanderthals were a primitive ancestor of man. Now they say they weren't, it was a separate branch. That means they got it wrong, so that means the theory of evolution is wrong, so that means god created everything.
Sorry folks, but science just doesn't work like that. Science is more flexible that conservative christianity. Scientists just update their knowledge to incorporate the new information. This could just mean acknowledging that a conclusion was wrong and changing it based on the new info, or can in some cases mean realising that a theory is wrong, and either improving it or replacing it. The world would be a lot better place if all religions were open to a similar evolution.