Hello,
I am ultra noob when it comes to electronics. I do have the basic knowledge, though I still don't understand the concept of the Ground Point. I would like to know if I builded a circuit like this (below) if the Arduino Nano would normally process the information or if any incidents such as "short circuits" or any disfunction on the "materials".
Grateful,
João.
I don't know anything about the Gas Sensor Board in your schematic. And, I don't know what the battery voltage is, so I can't say if it's "safe". The resistors should prevent excess current if something goes wrong, although the 100 Ohm resistor is not quite high enough if the end connected to the Gas Sensor gets shorted.
A short is simply a "connection". Usually, it implies an unwanted connection. For example, if you accidently short (directly connect) an output to ground or to the power supply, you can damage the device. Similarly, if you short two outputs together one can go "high" while the other goes "low" and the device can be damaged. A short is zero resistance, which means excess current can flow, possibly damaging something.
You can get similar results if you connect something with too-low of an impedance. For example, connecting an 8 Ohm speaker to one of the Arduino's outputs will cause excess current to flow.
Another common way to damage something is to over-voltage it. If you connect 12V to the 5V power-input (or any input/output pin), you may damage the device.
It's generally OK to short inputs together, and the normal operation of the reset button is to "short" the reset line to ground... So, it's not always a bad thing.
The opposite of a short is a "open". This generally means that there is no connection where there should be a conection. An open is far-less likely to damage something, since with no connection, no current can flow. Again, a switch or relay can intentionally open a connection.
I am ultra noob when it comes to electronics. ... any incidents such as "short circuits" or any disfunction on the "materials".
If you are new to electronics, you are probably going to "blow up" (electrically damage) some parts sooner or later. 
It happens to experienced engineers too! It's a bit "dangerous" to start-out playing around with expensive parts... If you take electronics in school, they will start you out with resistors & capacitors that are almost free, and work-up to chips that cost less than a dollar, before moving-up to microcontrollers and other more costly parts.
You seem to have the Gnd on the sensor connected to the wrong pin,
not the gnd pin.
As regards short circuits, you've already included good protection against
that = all those series Rs in the Arduino I/O lines. I typically use 220-330 ohms
in all my I/O lines.
I am using the MQ-3 sensor from Sparkfun. The idea of the resistors is simply to measure the voltage and contrast it with the regular voltage of 4.8 to 9 Volts (not sure which battery I'm using). By that determine the Blood Alcohol Concentration.
The different values of resistors are to obtain different amounts of voltage and determine the blood alcohol concentration with more precision.
I will change the A1 to GND, thank you.
I will also change it to 220?, 470? and 700?.
The Arduino Nano Board resists how much of current and voltage?
Is there any part that reacts to electricity that opens and closes, as if it was a door?
MQ-3 sensor from Sparkfun
In this case, it seems to me your schematic shown is completely wrong,
but I find the MQ3 datasheet to be almost indecipherable. 2 A's, 2 B's,
and 2 H's all being totally different signals. Who draws these things?
Gakk. I find the Sparkfun BOB schematic to also be bizarre.
I wouldn't be confident unless I had the part in front of me, so I could
take a DMM to it, and figure out what comes out what pin.
It looks like from the BOB shcematic that the 5V and Gnd pins are pretty
clear. However, what is the difference between the 2 A's?
In any case, it looks like at least 1 of the A outputs is "analog", so you
need to connect it to an Arduino A/D channel, not a digital channel.
It also looks like you need a sensing resistor to gnd, connected to one of
A outputs, but the MQ3 d/s calls it RS in one place and RL in the other,
so everything is very confusing.
And I don't see why you have those 100, 220 and 470 R's at all.
Good luck.
I agree with oric_dan(333) here. Your schematic is wrong.
BTW, this project sound interresting. I was planning to make a breath analyser using the Arduino. A device like that will be usefull, because I don't want to lose my drving licence / driving job.
First. When you are planning to build a project, you always download / get a datasheet. So I check and here it is : http://www.sparkfun.com/datasheets/Sensors/MQ-3.pdf
Did you read the datasheet ? And the sensor is only $5.00 anyway.
Let see... GND to gnd <-- That is easy
+5 V to H <-- H is heater @ 33 ohms --> 5 V / 33 ohms = 150 mA <-- bear that in mind
A1 + A2 = Connect them together and B1 + B2 = Connect together
A1-A2 OR B1-B2 connect to +5 V <-- your pick
Take A1-A2 OR B1-B2 to any analog pin <-- the left over to the analog pin
And connect a resistor of 47 K to 100 K between analog pin and gnd. <-- the analog pin you pick.
That it.
Program, test for a value, get drunk, test your breath. 
Why should I use the analog pin anyway, if all the pins of the MQ-3 are digital?
Is this more correct Techone?
Almost right...
A2 connection is wrong. Connect to +5 V
B1 connection is wrong. Connect to Analog 4 just like B2
GND is correct.
H1 is correct.
R1 is too low. A 100 K to 470 K
Check the datasheet of the MQ-3
Why should I use the analog pin anyway, if all the pins of the MQ-3 are digital?
The MQ-3 is NOT digital. It is an analog device. And the device need to be calibrated by varing RL ( from 100 K to 470 K ). And also in coding. <-- use Math to calibrated. You need to convert the analog value ( 0 - 1023 ) into Alcool Concentration. The datasheet graph of the device is not linear, more like a logaritm graph or some kind of frequency responce graph. So to calculate is going to be a "bit tricky".
Thank you a lot. I am ordering the parts tomorrow 
Now I got it. The whole problem was that I thought the pins were digital and I imagined the voltage was changed by the MQ3 according to the amount of alcohol. And actually it produced only different Analog Waves that are related to the alcohol concentration.
Got my self cleared up, here is the last modification you actually proposed to make, and I still kept one of the pins connected to the Digital because I would need to transmit later directly digital information.
