X-ray tubes are just a convenient way of producing x-rays. The tube contains a vacuum, which means that the accelerated electrons won't accidentally bump into any other particles and be scattered or...
Because the sound of X in English is that of a vowel sound. What this means is that you pronounce it "EX", with the sound of the "E" first. Ergo, one says an "ex". It's actually logical in this case...
X-rays were first discovered by Rontgen in 1895, he took and x-ray image of his wifes hand on the 8th of November that year. The first medically beneficial x-ray was taken on the 13th of January...