What is the difference between intensive margin and extensive margin in labor economics? What is the difference between intensive margin and extensive margin in labor economics, or general RBC model, where we talk about labor-hours supplied changing with extensive margin or intensive margin?
In Labor Economics, "Extensive margin" refers to "how many people work". "Intensive margin" refers to "how much a given number of people work, on average". To copy from a freely available recent study by Blundell, Bozio and Laroque 2011,


  "...we split the overall level of work activity into the number of individuals in work and the intensity of work supplied by those in
  work. This reflects the distinction between whether to work and how
  much to work at the individual level and is referred to, respectively,
  as the extensive and intensive margin of labour supply. At the
  aggregate level the former is typically measured by the number of
  individuals in paid employment and the later by the average number of
  working hours."


Evidently, it is an important distinction, especially when one wants to analyze changes in employment, measured say in total number of hours worked: Say, did they increase? Why? Because more people work, or because the same number of people work more? And if the answer is "both", then what portion is attributable to a change in the intensive margin, and what portion to a change in the extensive margin? (this is why the word "margin" is used).

Of course "intensity" as a word has a more general meaning, as for example the one mentioned in another answer here -it may be the case that people work "faster" and so in the same time interval, they produce more. Such changes are usually put under the concept of "changes in efficiency" (for example in Growth theory this is how they are called). In Labor Economics the terms have the meaning given above.