Evolution is the scientifically accepted system which explains how we are in our current form. The easiest way to explain evolution is by example. Say that Africa was full of short necked giraffes that all had to compete for the same leaves on trees as all other grazing creatures. Those giraffes in the herd that had longer necks would be able to reach more leaves. They would therefore be more healthy and live longer, having more kids. Those kids would naturally have long necks like their mommies and daddies. Over time, the short necked giraffes would die out, because they would not be as healthy as the long necked ones were. This results in long necked giraffes.

Creationism
Creationism argues that God created - in a single flash - all creatures of the earth exactly as they are today. Creationism says that no creature ever changed from that initial flash of creation. The argument against creationism is pretty clear - we ourselves have made new dog and cat breeds that exist in modern times, that did not exist 4,000 years ago during recorded history. That is why few people in modern times believe in Creationism. Instead, those who are opposed to Evolution believe in Intelligent Design.

Intelligent Design
In Intelligent Design, the theory is that animals and people DID change over the years, since we can clearly see that in skeletal and fossil records. However, Intelligent Design proponents say that these changes did not happen because of nature. They say that God deliberately charted the paths that caused various creatures to change and evolve over time. It was a deliberate guided pathway to reach the current ends.

Evolution is Only a Theory?

Creationists often make a big deal of evolution "only" being a theory. This demonstrations that creationists do not understand what the word theory means in science. Theory actually means "a group of concepts that together explain a phenomenon".

For example, the earth rotating around the sun is a "theory". It is a working set of different ideas (physics, cosmology, etc) that define a system. That theory is *composed* of many facts - the sun is a star, the sun has gasses at its core, etc. But you don't call a system of ideas a "fact".

To try to make this clearer to creationists, many scientists have begin calling evolution a "fact", because it is not in dispute by the worldwide body of science. Creationists simply do not understand the standard terms of science, to think that "theory" somehow means it is in doubt. There is no doubt about evolution. It is the accepted and proven method by which organisms change over time.

Evolutionary changes have been documented within the past 100 years, those changes are documented facts. For a creationist to deny evolution, they would have to deny the changes which have happened within our own lifetime.