Theories are explanations as to how things work. Think they sometimes get theory and hypothesis confused. 
A high percentage of theories start out as a hypothesis, with something being observed and with no knowledge of the mechanisms involved, a hypothesis will be formulated to test the observed phenomena. The hypothesis is considered the starting point, the theory the end point (though there's normally the reason someone has come up with the hypothesis in the first place that's the starting point, which could be considered an untested theory, or simply an idea or notion). It's only after the hypothesis has been proven and widely accepted it is really a fully fledged theory. the use of "new theories hypothesised as more evidence is found and knowledge expands" was pointing to the fact that existing theories lead to new hypotheses with additonal knowledge and understanding, and new theories result from these hypotheses, but just worded very badly.
It's the use of theory that is probably most misused as theories have to have undergone testing and been accepted. For example somebody might see somebody else get punched in the head. As a result they might then say to their friend "I have a theory that if I punch you in the head, it's going to hurt". Technically not a theory because it hasn't been tested (though there's a good chance we know it's going to be true so it's our idea or untested theory and starting point). So a hypothesis to test the idea that a punch in the head will hurt can be formulated (normally the null hypothesis that's tested). Once the testing has been completed and results confirm that it does actually hurt. the theory can be accepted. (it's a poor analogy, I know, but it's late and I couldn't think of a better one).
Science is always looking to improve and theories that are widely accepted can also lead to a new hypothesis as scientists look to poke holes in existing theories, possibly as a result of advances in science and new knowledge.
It's this continual re-testing of our sicentific knowledge that allows people such as creationsists to latch on to something and try to debunk it.
A good example of this is Darwin's (and Wallace's) theories on Evolution. At the time they came up with the theory, pretty much in parallel, science wasn't advanced enough to test much of the theory, other than basic examination of skeletal and fossil remains, basic taxonomy and observations of similarities/differences in species. The theory hadn't really undergone much rigorous scientific testing without the ability to fully study genetic information. At that time no genomes had been mapped and methods such as cladistics weren't around either. So new hypotheses have been formulated over time to further test the original theory and clarify it.
Most of what the creationsists try and debunk about evolution is based on Darwin's original theory and the flaws resulting from the lack of additonal knowledge at that time. They latch onto the holes poked in the theory as a result of new knowledge, testing and the work of modern day evolutionary biologists. They can't wait to shout about the flaws in Darwin's theory of evolution that scientists have found but they happily ignore the advances in knowledge and science that have lead to improvements in evolutionary theory. Some still look at this and try and debunk it because scientists continue to try and improve the theory and gain more knowledge.
All the while they totally ignore the fact that there is no scientific evidence to back up their alternate view of the origins of man, no tested hypothesis to allow the theory of God's creation to be accepted. Their acceptance of the existence of God is based purely on their belief that it is true.
