[XeTeX] Newbie's XeTeX questions

[Elof Ng]

In the document named "XETEX, the Multilingual Lion: TEX meets Unicode 
and smart font technologies", Kew mentioned the e-TEX’s typesetting 
engine, saying XeTeX is in fact based on it, but I found nothing about 
the e-TEX.

Is it same as PlainTeX?

I have briefly read the XeTeX Reference guid, Companion, they just 
introduce the topics related to the font, what about the others?

Is XeTeX composed same as PlainTeX?

Do I have to learn TeX first?