How do they produce air on the ISS? How do they provide air on the ISS?  I know it is produced by electrolysis but is that all they breathe, hydrogen and oxygen?  The air here on Earth contains other things like nitrogen.  Do the people on the ISS not need nitrogen?  Or do they take what is needed on each launch?  If so how much water do they take and how long does a certain amount of water last when used for air?  Seems like a whole lot of weight.  Now they are planning a yearlong trip.  How much water will they have to launch with?
 A: The ISS is using electrolysis, in the "Elektron" oxygen generator,

http://en.wikipedia.org/wiki/Elektron_(ISS)#Elektron

to separate water to hydrogen and oxygen. NASA claims that those devices don't work well and prefers bottled oxygen delivered from Earth which the Russians only consider a backup option. Another backup option are solid fuel oxygen generator canisters "Vika" producing oxygen from a liter of lithium perchlorate.

http://en.wikipedia.org/wiki/Vika_oxygen_generator

The atmosphere at the ISS has 101,300 Pa, just like on Earth, and contains all the right composition of gases we know from Earth. A typical astronauts' alternative is pure oxygen which is OK to breathe for humans but poses a higher fire risk.
The mass of the air the space station needs is rather modest. A typical person breathes 4-12 times a minute and one breath is 0.5-6.0 liters. Take 10 times 2 liters a minute, 20 liters a minute, 1,200 liters an hour, 30,000 liters a day. It may sound like a lot but 30,000 liters is just 30 kilograms or so and only 1/4 of it is "consumed" (oxygen), so what one needs is comparable to a few kilograms per day.
The Vika system above claims to give 1 person enough oxygen for 24 hours just from 1 liter of the lithium perchlorate (whose density is 2.4 kg/liter). To have enough oxygen for a 100-day mission, you have to deliver about the same mass of the lithium perchlorate for the person as his own weight. The counting is comparable with other sources, including the bottled oxygen (that occupies more room because the density is lower). The astronaut will need twice his mass of water to drink and one times his mass of food for those 100 days. Estimates. At any rate, what you need to deliver with the astronauts to allow them life for many months is themselves plus a small multiple of their mass in various food/water/air reserves.
See also a question here on cooling mechanisms and suits:

What kind of cooling mechanism could be used in outer space?

A: In addition to Luboš's answer regarding the O2/CO2 requirement, the N2 aspect isn't as much of a problem as you don't 'use' it like you do O2. 
Once you strip out the expired CO2 from the air and replace with O2 the N2 is still there as it was before, so you don't need to take up vast quantities of it from Earth. Which is rather lucky, as it obviously makes up the majority of the air we breathe.
A: Not sure I'm convinced by the decision to use N2/O2 to reduce fire risk.
Is fire much more of a problem in 0.2atm of pure O2 than in air?
The Apollo I fire was because some idiot put it under a 1.2Atm O2 partial pressure, even if you had added another 0.8atm of N2 it would still have been lethal.
If having a 1atm pressure is necessary for comfort (maintaining humidity, sinus issues etc) wouldn't you choose a different gas than N2? Perhaps He or Ar, it's not like cost is a big driver.
