The fermentation process of beer is composed of 3 phases: Lag, Active, and cleanup
The lag phase or growth phase is when the yeast consume the oxygen from aerating the wort and nutrients to multiply up to a healthy strong colony to begin fermenting the wort
Active fermentation is the time the yeast are actually consuming the sugars present to convert to alcohol. This is the time you see krausen development and lots of airlock activity
Cleanup is after active fermentation begins to subside and the yeast begin consuming their own fermentation by products as well as chew through the remaining sugars to achieve FG. This is the period of time that can take the longest and appear as though nothing is happening when in fact there is still activity.
Obviously this is a very basic explanation of the process. The lag/growth phase can take up to 72 hours, active can occur over a period of 3-5 days generally, sometimes less, sometimes more. The final phase can take an additional week or two depending on the strain, how well the wort was aerated, how fermentable the wort was, etc.