This is actually a repost from another discussion forum, but I thought it might be worthwhile bringing it over here. One of the problems I see in the Evolution/Creation debate is the rather ill-defined nature of the word 'evolution'. What follows is my attempt to break it down into individual parts:
Decent With Modification. The idea that all species alive on earth today derived from earlier species, changing by small steps each time. I consider this proven beyond any reasonable doubt. The fossil record unanimously supports it, DNA sequencing unanimously supports it.
Single Common Ancestor. The idea that all life today shares a single common ancestor. Certain similarities in DNA across all life hugely supports this idea. That the exact same mechanism could arise independently multiple times is radically unlikely. The only possible exception is Viruses, who are only loosely categorised as life anyway (can only reproduce with help and all that), but even their RNA has a few tell-tale hints in it.
Natural Selection. The idea that the major mechanism responsible for this Decent With Modification is the principle of Natural Selection. That Natural Selection occurs is almost unarguable, and research such as the infamous speckled moths experimentally verifies it. Unfortunately it is massively harder to demonstrate that it is responsible for the observed changes over time. What we can do is model the potential of Natural Selection mathematically and using computers. And indeed these both support the idea of NS being sufficiently capable. Since no-one has an alternative mechanism capable of this action, I accept NS as being the driving force behind Decent With Modification. While other actions almost certainly have an effect (genetic drift, for example), they are not responsible for the increase in Fitness, and Complexity, exhibited by life on earth.
Historical Reconstruction. Working out which ancestor links to whom; tracing out the intricate paths of life on earth. This is the weakest part of evolutionary theory, while we can establish this in broad sweeps for most life and fairly precisely for those few species we have a good record for, the fossils record isn't complete enough to cover all life, and it is almost impossible to tell whether a fossil is a direct ancestor or a 'cousin' of that ancestor.
Things which are not evolution (but are often lumped with it). Abiogenesis, the big bang, plate tectonics, solar system formation, radioactive dating, etc. These range from speculation (abiogenesis), to certain fact but unclear mechanism (big bang) to certain fact and clear mechanism (radioactive dating, and plate tectonics). But I shan't discuss them in detail here.