What exactly are Trojans

I know this is definitely a stupid question, but what exactly are trojans, what do they do and why should i fear them.

I have Mcafee Enterprise 8 installed, which i chose largely because i have been told is much more effective than norton at dealing with Trojans, worms and backdoors, i thought great i should get this, but i havent a clue what Worms backdoors and more importantly trojans are. I also have a2 and trojen Hunter installed, as i have been advised to do so, but i do not know what it is helping with.

i have searched them all on yahoo but i couldnt seem to follow the information i got, all they seem to like to say for trojans is they are named after the battle of Troy where a wooden horse was built which tricked the trojans.

But other than that i cant seem to follow the full description of them as it gets too technical.

So can either someone please explain in lamens terms what they are, or could you provide a link to a web page designed for idiots like myself which are good at describing trojans, worms etc in simple lamens terms.

Thanks, and sorry for the stupid question

Thank you