X is the 24th letter of the English alphabet. Its name sounds like 'ecks' in English and 'equis' in Spanish.
The letter has several sounds: /ks/ and /gz/ in English (/z/ at the beginning of words: Xena, xylophone). In Spanish it also represents /x/ (sounds like h or the ch-sound in loch, depending on your dialect). In Portuguese it can sound like /S/ (a sh-sound).
The letter's shape is borrowed from what we know of now as the Greek letter chi; for a very long time in Greek this letter was first pronounced as an aspirated 'k', then fricativized, but before the alphabets were unified, and different letters stood for different things in different places, the Etruscans (who learned the alphabet from the Greeks) had borrowed a chi that sounded like /ks/. When the Etruscans brought the alphabet to the Romans, the sound remained. And that's why Latin X doesn't look like Greek Ξ!
X is not a very common letter, though you can use it if you want to be exotic. In Latin the letter X seems to be used as an abbreviation for cs (/ks/ sound) wherever possible, but in English we tend to be more conservative about such things.