Units at Stanford are a metric assigned to each course that roughly correspond to the amount of class time required per week. (That is, 3 unit classes meet for 3 units per week.) Units are important for students because there are limits on the number of units allowed per quarter and also particular unit requirements for degree programs.
Relation to amount of workEdit
Although the number of units a class grants is intended to indicate the amount of work required for the class, many question how significant the correlation is. For 1 unit classes, very little work is ever necessary--most times only attendance. (I do not know anything about 2 unit classes). Many classes designated 3 will be far more involved than 5 unit classes. Often this is very true when comparing advanced classes in engineering to introductory classes like IHUM.
Variable number of units for the same classEdit
Certain classes offer an option to take the class for varying numbers of units. This can be deceptive, since this practice is often in order to accommodate graduate students' lesser unit caps and does not actually denote the ability to take an "easier" version of the class. In other cases, a course offered for fewer units may demand less of the students taking it for the reduced number.