Hi all! Recently, I've ordered Arduino Uno R3. I'm very very new to electronics as a whole and I'm curious if I can use (when the board arrives) one universal AC->DC adapter to provide power for this board. So this is how the adapter looks like:
By specification the adapter output is 2.5A allegendly on all modes. There is no detail information and I have no multimeter to check the actual amperage on each voltage 
so I want to know if I plug it to board it doesn't burn 
The Arduino will only draw as much current as it needs; a more capable power supply will not hurt it.
There is a protection diode on the input of the barrel jack. So even if plus and minus are swapped on the barrel jack, the Uno R3 is protected.
Per sterretje, you're misunderstanding how voltage and current work. This is a very common point of confusion, and I won't belabor the topic here - questions like yours are asked often, and there are also a great many resources online that will explain this better than I ever could.
Watch out - almost all of those kind of wallwarts, with the voltage adjust switch on them, are not properly regulated and the output voltage can be much higher under a light load. There's no technical reason they couldn't do such an adjustable wallwart right, it just seems that the bad outnumber the good. So I recommend staying away from them - though you're probably okay if you put it into the barrel jack. Definitely don't set it for 5v and connect it to the 5v pin!
If there isn't a reason not to do this in your case, your first choice should be a USB charger, and a USB cable plugged into the USB port on the Uno.
Second choice is 7~12v on the barrel jack (center positive), lower voltages better than higher ones.
Third choice is 5v into the 5v pin - do not connect it to the computer while it's wired like this.
Per sterretje, you're misunderstanding how voltage and current work. This is a very common point of confusion, and I won't belabor the topic here - questions like yours are asked often, and there are also a great many resources online that will explain this better than I ever could.
If so, it would be nice if you suggest an article that "fits" in this case of misconception. After reading some ones it looks like with no potential difference (V) there is no current at all. I think, that is why the voltage is the main parameter in power supplies. They create a difference, but futher flow of charge will depend on the conductor/wire of a circuit. It seems reasonable that @sterretje said "Arduino will only draw as much current as it needs". And that's why the adapter specify the only maximum amperage. Doesn't it?...
Watch out - almost all of those kind of wallwarts, with the voltage adjust switch on them, are not properly regulated and the output voltage can be much higher under a light load. ... So I recommend staying away from them - though you're probably okay if you put it into the barrel jack. Definitely don't set it for 5v and connect it to the 5v pin!
Thank you! Good advice I will take it into account. Anyway I've found several DC adapters that will fit into my cases, so now I don't need this one. Great!
Yep to your question on quoting maximum amps... except for current devices like LEDs, by and large things draw what they need. An led is pretty much a short circuit once it gets enough volts though, and need a series resistor to keep the current at bay.
On the wall wart voltage, I posted earlier today that mine, very similar to yours, is over 10V unloaded on the 6V setting. I also noticed I wrote on it that 4.5V gives 7V and 7.5V is over 12V.
