Primary fermentation is where the bulk of the magic happens, the beer will be fermenting away, with the yeasts consuming the sugar and producing heaps of CO2 and alcohol. This is also the stage at which the majority of the flavour of the beer is decided, and as such is also the area where the flavour can be messed up.
As mentioned before, the yeast likes to operate in a certain temperature window. Operate above this and you will get undesirable flavours, and you might even kill the yeast. Operate below and fermentation will take longer, if it happens at all. Generally it is better to err on the side of slightly too cold than too hot.
Think of it this way, if you had someone producing some work, and the temperature is the amount of energy they have. Too much energy and the work gets sloppy and they might even have a heart attack if they have too much. On the other hand if they don’t have enough energy it will take them longer to do the work, although they will still complete the work to the same standard, unless of course they fall asleep. With the quality of the work relating to the flavour of the beer.
You can measure the progress of fermentation using a hydrometer to measure Specific Gravity (SG) of the beer. SG is related to the density of the liquid. Sugar makes the SG higher, while alcohol lowers it, hence as fermentation converts sugar to alcohol the Gravity will drop.
People often use the air lock bubbling as a sign of fermentation and this can often lead to people bottling before they should, this is compounded by the ‘instructions’ received with some kits that state that fermentation can be over within a matter of days. In reality you want to leave the beer in primary fermentation for a long while longer than 4-6 days, 3 weeks is more along the lines of what is required. The bulk of the fermentation will be over well before this but the extra time helps some of the crud settle out and the beer should mature in flavour and become more clear, leading to a better beer. You don’t have to worry about leaving it for too long in primary fermentation, there have been stories of people leaving their beer for several months at a time with no adverse effects, although typically about a month is probably the longest you would want to leave a standard beer, but once you have measure three consecutive days at the same Specific Gravity you are generally good to go, but use common sense. If, for example, days 2,3 and 4 of fermentation are all the same, chances are your beer hasn’t finished fermenting and the yeast is likely to be inactive.
On the topic of fermentation, the key difference between lager yeasts and ale type yeasts is the fermentation. As mentioned before lager yeasts are ‘bottom fermenting’ and ale yeasts are ‘top fermenting,’ with lager yeasts generally preferring a colder fermentation temperature than ale yeasts. Due to this lower temperature the lager yeasts will typically take longer to ferment, and generally require a refrigerator to keep the temps down and constant, explaining why lagers are less popular with home brewers. The “lager” kit I have doesn’t use lager yeasts, hence calling it “lager” rather than lager.
Primary fermentation is finished when the Specific Gravity (SG) of the beer is constant over the course of a few days (typically 3 days), which indicates no more sugar is being converted to alcohol. You can use this reading of SG (known as Final Gravity, FG) in conjunction with the Original Gravity (OG) you measured at the beginning to calculate the alcohol content of the beer using the following formula:
( OG – FG ) * 131 = ABV (in %)
Example: OG = 1.050, FG = 1.010
(1.050-1.010)* 131 = 5.24% ABV