The old Lovely Cyborg had about four different sections on it, all with different names, devoted to disproving misconceptions about #18. But like I said in the Basics section, that was back before the whole series had been translated into English. That was also before I could read Japanese. So let's do this over again, in a condensed version...

Things We Know About #18

Almost all of the following statements are backed up with evidence found in the Dragon Ball canon, such as the manga and Daizenshuu (the DB encyclopedias). Much of that information, which is scanned and translated by myself, can be found in the Cyborg or Android section. Rather than repost it all here, if you don't believe what's written on this page, check that one out.

She used to be a human

I think this is common knowledge by now, even among new fans (thank goodness). Her and her twin brother, #17, used to be humans, and were taken against their will by Dr. Gero and forcefully turned into cyborgs. While something like that sounds horribly traumatic, Eighteen's reproductive system was left in tact, which is how she was able to give birth to her daughter, Marron. What's funny is that before Marron was born, Goku had no idea she wasn't a robot and got all bent out of shape when he first saw the little girl.

#17 and #18 were actual twins before they were cyborgs

The twins are referred to a few times as 双子 futago, Japanese for maternal twins. They're not stated to be identical twins, but they sure do look alike anyhow.

She is the strongest female character in the series

I'm afraid this is stated as cold hard fact in Daizenshuu 7, although it does state that she is the strongest among the married women （奥さん）, which technically leaves Pan from non-canon Dragon Ball GT. But really though?

She will not visibly age

I may have added "visibly" to age, but Daizenshuu 7 states that she is dropdead gorgeous and will not age. So yeah, I'm inclined to think that she's going to look like a hot teen for the rest of her life (which is amply explored in all the 18xKuririn hentai doujinshi...).

She has infinite energy

How would you feel if you knew you'd never feel fatigue? Yes, it's true. Her and #17 like to brag about how they don't get tired, thanks to Dr. Gero's decision to modify their bodies to accept eternal energy reactors. Apparently, though, these reactors are some heavy duty business that gives them so much power that it's difficult to control them, or so Dr. Gero believed (he changed energy type in his next model after). But if you were kidnapped and violated by a madman like they were, would you comply with the lunatic once you were superhuman?

She is the older twin

I don't think this ever really mattered, but according to the all-powerful Daizenshuu, Eighteen popped out of the womb before her brother.

Things that fans made up about #18

This title sounds kind of harsh, so I should soften it by adding that the following notions have also been contested issues in the heyday of Dragon Ball fandom (late 90s, early 2000s).

She is part human, part android

This always makes me cringe. Let it be known far and wide that an android is a robot which resembles a human! Robots initially need outside programming in order to operate. Eighteen is a cyborg, one that we know to be enhanced with mostly bio-organic materials and not electromechanical parts. While other cyborgs in real life and in fiction can have electromechanical enhancements in the form of prosthetics and such, this does not count as being "part android", based solely on the fact that an android is a robot. Humans are not robots. But humans can incorporate electromechanical systems into their bodies, as cyborgs, which provide feedback to their native system.

If you insist on saying Eighteen is part human, part something, say "partially enhanced". And then go read the Cyborg or Android section.

She was turned into a human by Shen Long.

#18 had a self-destruct mechanism planted inside her. At first, the Z gang asked Shen Long to turn her and her brother back into regular humans, but this was beyond the dragon's power. As the next best thing, he removed the mechanisms. Hurray!

She has no emotions

Okay, so she's kind of an apathetic enternally teenaged jerk who has a hard time expressing feelings in a passionate way (just remember the way she freaked out when Kuririn asked in #17 was her boyfriend), but this doesn't mean she has no feelings! This myth is based off the stereotype that cyborgs are beings that have been robbed of their humanity, or have crossed the boundary into something less human. This may be true for some cyborgs, but our favorite blonde definitely has her moments, like getting way pissed at Mr. Satan for trying to screw her out of money, and being scared for her life when Cell is about to kill her.

She feels no pain

This is something that popped up in a lot of fanfics and role-play logs back in the day. It was perceived that since #18 didn't age and didn't get tired, she also couldn't feel pain. This is never stated in any source material, so it's not true. I suppose this myth stemmed from the way she fought Vegeta — she was stronger than him at the time, and so his blows didn't affect her, and her attitude was so apathetic and straight-faced, that it's easy to see how fans could think she feels no pain.

She is the same power level as #17

It's stated in the Daizenshuu that #17 is a little more powerful than her. Just by a little. In the Android ABC chart (did you read the whole Cyborg or Android section?), it's noted that #17 was made too powerful, and as a result, #18's power was suppressed lightly in hopes that she would be more compliant. (psyyyych!)

If #18 continued to train and didn't settle down, she might be as strong as Vegeta or Goku.

Someone asked me this once, and I thought about it for a moment before deciding it was not possible. #18 is essentially a human, and even without cybernetics she could conceivably only grow as powerful as the strongest humans (e.g. Kuririn, Tenshinhan), who are far below the saiyans in power. The fact is that she does have some pretty badass cybernetic enhancements, of a level that makes her superior to super saiyan Vegeta at her debut. #17 is notably as powerful as the Kami/Piccolo fusion when he first fights, as well. But the reason why #17 and #18 gained the upper hand in their fights was more due to the fact that they don't get tired than their power being that much greater.

Still, if that's how strong the twins were at their debuts, one must wonder whether or not they had been interested in martial arts before they were modified. My fan feeling is that they didn't, since it's noted they were notorious delinquents in their former lives, so all that power was concievably due solely to their cybernetic enhancements. #18's power level does continue to rise throughout the series, however, but this is probably because we see her training with Kuririn on Kame House island. It's arguable that she could train to the max of her cybernetic limits, but something tells me that they can only go so far when compared to a super saiyan, especially one of the third level, fused with another one (ahem SSJ3 Gogeta anyone).

Videl or Pan are stronger than her

A hotly contested issue, but one settled yet again in Daizenshuu 7, where it is stated that #18 is strongest of the ladies. Actually, the translation is strongest of the 奥さん okusan, meaning married women, which includes Videl, but not Pan. Then again, does anyone really believe Pan is that strong in the first place, or is it the typical rabid fan that continues to debate this?