You will see IB DP courses listed as either "SL" or "HL": 

• SL stands for "Standard Level." SL courses involve either one or two years of study before testing.  

• HL stands for "Higher Level." HL courses always involve two years of study before testing.

The difference between SL and HL in a given subject is a matter of depth and breadth: HL courses cover everything done in an SL course, plus an additional set of extension topics. Depending on the subject area, HL courses often require additional skills or fluency. 

In Biology and Physics, students must take both years of the course sequence in order to test either SL or HL. In other areas, students take either a one-year SL course or a two-year HL course sequence before testing. Courses in English, Social Studies, Math, and the Arts work like this. All language acquisition classes test in 12th grade based on the level they have achieved (year 4 is SL, year 5 or higher is HL).