As maddawg said above xanax is a benzo, it is a Central Nervous System depressant and is prescribed for anxiety, as well as sleeping disorders and certain mental disorders and mood disorders occasionally. It is really only handed out a lot in the U.S., canada and western europe. In other places doctors tend to be more cautious about prescribing it although there may be more generic/street xanax available elsewhere - in the U.S. there is also a thriving market for street-pressed "bars" which are just alprazolam (the active ingredient in xanax) or a similar benzo pressed into pills that are supposed to look a lot like the legit prescriptions.
Dosage is typically 0.5mg-2mg when prescribed, although some rec doses can exceed even 10mg. For your first few times I would not take more than 3mg in one dose (1.5 white bars or 3 blue ovals if those are the pills you have, I don't know the dosage for other pills off the top of my head).
Side effects: memory loss/black outs, slowed reaction time, impaired motor skills, dizziness, sluggishness, impaired judgement, passing out, physical and mental addiction.
The ones you need to watch for the most (besides addiction forming after frequent use) are the memory loss, impaired judgement and blacking out/passing out - a common story/pattern of use for many people is "Yeah man yesterday I bought 10 bars, popped 2 of them, an hour later decided to take another 2, then passed out and woke up the next morning with all 10 of them gone and a bad cut/bruise on my (name a body part).
Just take it slow, don't overuse them and don't drive or do other stupid stuff on them and you'll be fine.
Hope that answered all of your questions.
